U.S. patent application number 16/660160 was filed with the patent office on 2021-04-22 for fabrication of covox composite thin film electrode via single step aacvd.
This patent application is currently assigned to King Fahd University of Petroleum and Minerals. The applicant listed for this patent is King Fahd University of Petroleum and Minerals. Invention is credited to Muhammad Ali EHSAN, Abdul REHMAN, Abbas Hakeem SAEED.
Application Number | 20210115578 16/660160 |
Document ID | / |
Family ID | 1000004444178 |
Filed Date | 2021-04-22 |
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United States Patent
Application |
20210115578 |
Kind Code |
A1 |
REHMAN; Abdul ; et
al. |
April 22, 2021 |
FABRICATION OF CoVOx COMPOSITE THIN FILM ELECTRODE VIA SINGLE STEP
AACVD
Abstract
A CoVO.sub.x composite electrode and method of making is
described. The composite electrode comprises a substrate with an
average 0.5-5 .mu.m thick layer of CoVO.sub.x having pores with
average diameters of 2-200 nm. The method of making the composite
electrode involves contacting the substrate with an aerosol
comprising a solvent, a cobalt complex, and a vanadium complex. The
CoVO.sub.x composite electrode is capable of being used in an
electrochemical cell for water oxidation.
Inventors: |
REHMAN; Abdul; (Dhahran,
SA) ; SAEED; Abbas Hakeem; (Dhahran, SA) ;
EHSAN; Muhammad Ali; (Dhahran, SA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
King Fahd University of Petroleum and Minerals |
Dhahran |
|
SA |
|
|
Assignee: |
King Fahd University of Petroleum
and Minerals
Dhahran
SA
|
Family ID: |
1000004444178 |
Appl. No.: |
16/660160 |
Filed: |
October 22, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/073 20210101;
H01M 4/485 20130101; C01G 31/006 20130101; H01M 4/525 20130101;
C01G 51/42 20130101; C25B 1/04 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C01G 31/00 20060101 C01G031/00; H01M 4/485 20060101
H01M004/485; C01G 51/00 20060101 C01G051/00; H01M 4/525 20060101
H01M004/525; C25B 1/04 20060101 C25B001/04 |
Claims
1: A composite thin film electrode, comprising: a CoVO.sub.x layer
having an average thickness of 500 nm-5 .mu.m in contact with a
substrate, wherein the CoVO.sub.x layer comprises amorphous
CoVO.sub.x having a Co:V molar ratio in a range of 1.0:1.2-1.5:1.0,
and wherein the substrate is a transparent conducting film.
2: The composite thin film electrode of claim 1, wherein the
CoVO.sub.x layer is porous with a pore size in a range of 2-10
nm.
3: The composite thin film electrode of claim 1, which has an
electrochemically active surface area in a range of 12-22
mF/cm.sup.2.
4: The composite thin film electrode of claim 1, wherein the
CoVO.sub.x layer consists essentially of amorphous CoVO.sub.x.
5: The composite thin film electrode of claim 1, wherein the
CoVO.sub.x layer has an O:Co molar ratio in a range of 4:1 to
9:1.
6: The composite thin film electrode of claim 1, wherein the
transparent conducting film is selected from the group consisting
of fluorine-doped tin oxide, indium tin oxide, aluminum-doped zinc
oxide, gallium-doped zinc oxide, indium zinc oxide, indium zinc tin
oxide, indium aluminum zinc oxide, indium gallium zinc oxide,
indium gallium tin oxide, and antimony tin oxide.
7: A method of making the composite thin film electrode of claim 1,
the method comprising: contacting an aerosol with the substrate to
deposit the CoVO.sub.x layer to form the composite thin film
electrode, wherein the aerosol comprises a carrier gas, and a
cobalt complex and a vanadium complex dissolved in a solvent, and
wherein the substrate has a temperature in a range of
450-550.degree. C. during the contacting.
8: The method of claim 7, wherein the cobalt complex and the
vanadium complex each independently comprise at least one ligand
selected from the group consisting of acetylacetonate, acetate
ligand, trifluoroacetate, isopropanol, and tetrahydrofuran.
9: The method of claim 7, wherein the cobalt complex is Co(II)
acetylacetonate, and the vanadium complex is V(III)
acetylacetonate.
10: The method of claim 7, wherein before the contacting, the
aerosol consists essentially of the carrier gas, the cobalt
complex, the vanadium complex, and the solvent.
11: The method of claim 7, wherein a weight ratio of the cobalt
complex to the solvent in the aerosol, and/or a weight ratio of the
vanadium complex to the solvent in the aerosol is in a range of
1:1,000-1:5.
12: The method of claim 7, wherein the aerosol is contacted with
the substrate for a time period of 10-30 min.
13: The method of claim 7, wherein during the contacting, the
carrier gas has a flow rate in a range of 20-250 cm.sup.3/min.
14: An electrochemical cell, comprising: the composite thin film
electrode of claim 1; a counter electrode; and an electrolyte
solution in contact with both electrodes.
15: The electrochemical cell of claim 14, wherein the composite
thin film electrode has an overpotential in a range of 270-335 mV
at a current density of 9-11 mA/cm.sup.2.
16: The electrochemical cell of claim 14, wherein the composite
thin film electrode has a current density of 1.0-10.0 mA/cm.sup.2
when the electrodes are subjected to a bias potential of 1.45-1.55
V.
17: The electrochemical cell of claim 14, wherein the electrolyte
solution comprises water and an inorganic base having a
concentration of 0.1-1.0 M.
18: The electrochemical cell of claim 14, wherein the composite
thin film electrode has a mass activity in range of 38-50 A/g at a
potential of 350 mV.
19: A method for decomposing water into H.sub.2 and O.sub.2, the
method comprising: subjecting the electrodes of the electrochemical
cell of claim 14 with a potential of 0.5-2.0 V.
20: The method of claim 19, further comprising separately
collecting H.sub.2-enriched gas and O.sub.2-enriched gas.
Description
STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTORS
[0001] Aspects of this technology are described in an article
"Direct Deposition of Amorphous Cobalt-Vanadium Mixed Oxide Films
for Electrocatalytic Water Oxidation" by Muhammad Ali Ehsan, Abbas
Hakeem Saeed, Muhammad Sharif, and Abdul Rehman, in ACS Omega,
2019, 4, 12671-12679, DOI: 10.1021/acsomega.9b01385, which is
incorporated herein by reference in its entirety.
STATEMENT OF ACKNOWLEDGEMENT
[0002] This project was prepared with support from the Center of
Excellence in Nanotechnology (CENT) at King Fand University of
Petroleum & Minerals (KFUPM) and from the Deanship of
Scientific Research (DSR) at KFUPM: Project no. IN161012.
BACKGROUND OF THE INVENTION
Technical Field
[0003] The present invention relates to a method of making a
CoVO.sub.x composite thin film electrode that is capable of
electrocatalytic water splitting.
Description of the Related Art
[0004] The "background" description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent it is described in
this background section, as well as aspects of the description
which may not otherwise qualify as prior art at the time of filing,
are neither expressly or impliedly admitted as prior art against
the present invention.
[0005] Water splitting reactions for generating and storing clean
energy in the form of hydrogen may fulfill rising global energy
demands while mitigating ever-increasing environmental concerns.
See Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D. J.;
Higashi, M.; Kong, D.; Abe, R.; Tang, J., Mimicking Natural
Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme
Water Splitting Systems. Chemical Reviews 2018, 118 (10),
5201-5241; and Chu, S.; Cui, Y.; Liu, N., The path towards
sustainable energy. Nature Materials 2016, 16, 16, each
incorporated herein by reference in their entirety. A bottle neck
here is the four electron oxygen evolution reaction (OER), which
due to its sluggish kinetics and high overvoltage, requires highly
active catalytic materials for an economically viable rate of
reaction. See Reier, T.; Oezaslan, M.; Strasser, P.,
Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt
Catalysts: A Comparative Study of Nanoparticles and Bulk Materials.
ACS Catalysis 2012, 2 (8), 1765-1772; Reier, T.; Nong Hong, N.;
Teschner, D.; Schlogl, R.; Strasser, P., Electrocatalytic Oxygen
Evolution Reaction in Acidic Environments Reaction Mechanisms and
Catalysts. Advanced Energy Materials 2016, 7 (1), 1601275; and
Kumar, A.; Ciucci, F.; Morozovska, A. N.; Kalinin, S. V.; Jesse,
S., Measuring oxygen reduction/evolution reactions on the
nanoscale. Nature Chemistry 2011, 3, 707, each incorporated herein
by reference in their entirety. At the same time, these materials
need to be robust, efficient, and facile to be produced. Current
benchmarks for water oxidation are IrO.sub.2 and RuO.sub.2, but
these are based on scarce and costly noble metals, and thus
replacing them with earth abundant materials is an active area of
research. See McCrory, C. C. L.; Jung, S.; Peters, J. C.;
Jaramillo, T. F., Benchmarking Heterogeneous Electrocatalysts for
the Oxygen Evolution Reaction. Journal of the American Chemical
Society 2013, 135 (45), 16977-16987; Lyons, M. E. G.; Brandon, M.
P., A comparative study of the oxygen evolution reaction on
oxidised nickel, cobalt and iron electrodes in base. Journal of
Electroanalytical Chemistry 2010, 641 (1), 119-130; Faber, M. S.;
Jin, S., Earth-abundant inorganic electrocatalysts and their
nanostructures for energy conversion applications. Energy &
Environmental Science 2014, 7 (11), 3519-3542; and Roger, I.;
Shipman, M. A.; Symes, M. D., Earth-abundant catalysts for
electrochemical and photoelectrochemical water splitting. Nature
Reviews Chemistry 2017, 1, 0003, each incorporated herein by
reference in their entirety.
[0006] While the approach to develop these electrocatalysts mostly
remains empirical, some attempts to explore theoretical guidelines
have also been made. See Grimaud, A.; May, K. J.; Carlton, C. E.;
Lee, Y. L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y., Double
perovskites as a family of highly active catalysts for oxygen
evolution in alkaline solution. Nat Commun 2013, 4, 2439; and
Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.;
Shao-Horn, Y., A perovskite oxide optimized for oxygen evolution
catalysis from molecular orbital principles. Science 2011, 334
(6061), 1383-5, incorporated herein by reference in their
entirety.
[0007] In one such purely descriptor approach, the intrinsic
activity of the mixed metal oxide films is correlated to the M-OH
bond strengths using volcano plots. See Morales-Guio, C. G.;
Thorwarth, K.; Niesen, B.; Liardet, L.; Patscheider, J.; Ballif,
C.; Hu, X., Solar Hydrogen Production by Amorphous Silicon
Photocathodes Coated with a Magnetron Sputter Deposited Mo.sub.2C
Catalyst. J Am Chem Soc 2015, 137 (22), 7035-8, incorporated herein
by reference in its entirety. The outcome of this approach is quite
debatable as it neither describes the physical origin nor the
nature of the active sites in the metal oxide films. See Stevens,
M. B.; Trang, C. D. M.; Enman, L. J.; Deng, J.; Boettcher, S. W.,
Reactive Fe-Sites in Ni/Fe (Oxy)hydroxide Are Responsible for
Exceptional Oxygen Electrocatalysis Activity. J Am Chem Soc 2017,
139 (33), 11361-11364; and Friebel, D.; Louie, M. W.; Bajdich, M.;
Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.;
Weng, T.-C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Norskov,
J. K.; Nilsson, A.; Bell, A. T., Identification of Highly Active Fe
Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting. Journal
of the American Chemical Society 2015, 137 (3), 1305-1313, each
incorporated herein by reference in their entirety. However, it
approximates the superior catalytic activity of films like
NiFeO.sub.x and CoFeO.sub.x, explaining that Ni and Co are located
on different branches of the volcano plot as compared to Fe,
thereby benefiting from the balancing of M-OH bond strengths for
higher catalytic activity. Even better performance has been shown
by CoVO.sub.x catalysts, especially ones having amorphous
character, with V sitting in the same branch of volcano plot as Fe
while Co sits at exactly opposite branch, as depicted in FIG. 1.
See Liu, J.; Ji, Y.; Nai, J.; Niu, X.; Luo, Y.; Guo, L.; Yang, S.,
Ultrathin amorphous cobalt-vanadium hydr(oxy)oxide catalysts for
the oxygen evolution reaction. Energy & Environmental Science
2018, 11 (7), 1736-1741; Thorat, G. M.; Jadhav, H. S.; Roy, A.;
Chung, W.-J.; Seo, J. G., Dual Role of Deep Eutectic Solvent as a
Solvent and Template for the Synthesis of Octahedral Cobalt
Vanadate for an Oxygen Evolution Reaction. ACS Sustainable
Chemistry & Engineering 2018, 6 (12), 16255-16266; and Xing,
Z.; Wu, H.; Wu, L.; Wang, X.; Zhong, H.; Li, F.; Shi, J.; Song, D.;
Xiao, W.; Jiang, C.; Ren, F., A multifunctional vanadium-doped
cobalt oxide layer on silicon photoanodes for efficient and stable
photoelectrochemical water oxidation. Journal of Materials
Chemistry A 2018, 6 (42), 21167-21177, each incorporated herein by
reference in their entirety.
[0008] Thus, a recent trend is the straightforward, low
temperature, and fast fabrication of Co--V mixed oxide films having
amorphous character to provide abundant defects in a distinctive
molecular structure. See Chakrapani, K.; Bendt, G.; Hajiyani, H.;
Lunkenbein, T.; Greiner, M. T.; Masliuk, L.; Salamon, S.; Landers,
J.; Schlogl, R.; Wende, H.; Pentcheva, R.; Schulz, S.; Behrens, M.,
The Role of Composition of Uniform and Highly Dispersed Cobalt
Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis. ACS
Catalysis 2018, 8 (2), 1259-1267; and Peng, X.; Wang, L.; Hu, L.;
Li, Y.; Gao, B.; Song, H.; Huang, C.; Zhang, X.; Fu, J.; Huo, K.;
Chu, P. K., In situ segregation of cobalt nanoparticles on VN
nanosheets via nitriding of Co.sub.2V.sub.2O.sub.7 nanosheets as
efficient oxygen evolution reaction electrocatalysts. Nano Energy
2017, 34, 1-7, each incorporated herein by reference in their
entirety. Accordingly, Liardet et al synthesized amorphous
electrocatalysts based on Co--V mixed oxides with different
metallic ratios while approximating their position on the volcano
plots, and showed the highly active nature of the resulting
materials (e.g., a-Co.sub.0.50V.sub.0.42O.sub.x) when deposited
over glassy carbon electrodes and nickel foams. See Liardet, L.;
Hu, X., Amorphous Cobalt Vanadium Oxide as a Highly Active
Electrocatalyst for Oxygen Evolution. ACS Catalysis 2018, 8 (1),
644-650, incorporated herein by reference in its entirety. Liu et
al has also shown a similar catalytic activity of amorphous Co--V
(hydroxy)oxide ultrathin films when supported on gold foams.
[0009] Typically, two strategies are implemented in the rational
design of amorphous catalytic materials: (i) a solid-state reaction
(SSR) route using pure metals or metal salts, and (ii) wet
chemistry synthetic methods such as hydrothermal synthesis or
co-precipitation techniques. See Schmalzried, H., Solid-State
Reactions. Angewandte Chemie International Edition in English 1963,
2 (5), 251-254; Jiang, X.; Zhang, T.; Lee, J. Y., A Polymer-Infused
Solid-State Synthesis of a Long Cycle-Life
Na.sub.3V.sub.2(PO.sub.4).sub.3/C Composite. ACS Sustainable
Chemistry & Engineering 2017, 5 (9), 8447-8455; Liardet et al.
(2018); Kim, J. S.; Kim, S. Y.; Kim, D. H.; Ott, R. T.; Kim, H. G.;
Lee, M. H., Effect of hydrothermal condition on the formation of
multi-component oxides of Ni-based metallic glass under high
temperature water near the critical point. AIP Advances 2015, 5
(7), 077132; Liu et al. (2018); and Dolla, T. H.; Billing, D. G.;
Sheppard, C.; Prinsloo, A.; Carleschi, E.; Doyle, B. P.; Pruessner,
K.; Ndungu, P., Mn substituted Mn.sub.xZn.sub.1-xCO.sub.2O.sub.4
oxides synthesized by co-precipitation; effect of doping on the
structural, electronic and magnetic properties. RSC Advances 2018,
8 (70), 39837-39848, each incorporated herein by reference in their
entirety. The SSR routes are limited by a high temperature
processing and long reaction times because of the long diffusion
distances. See Fister, L.; Johnson, D. C., Controlling solid-state
reaction mechanisms using diffusion length in ultrathin-film
superlattice composites. Journal of the American Chemical Society
1992, 114 (12), 4639-4644, incorporated herein by reference in its
entirety. Still, the procedure has a lesser control on the size and
morphology of the final product. Typically, SSR for cobalt vanadate
is performed at high temperatures of >720.degree. C. for as long
as 40 h using vanadium oxides and hydrated cobalt oxalates. On the
other hand, wet chemical methods provide controllable synthesis and
intriguing morphologies with product formation occurring at
relatively low temperatures, yet require long reaction times and
expensive instruments such as high pressure reactors. Further
adding to this laborious work is the coating of resulting products
onto substrates in separate manipulation steps. Therefore, the
scale up of the final product becomes a limiting factor for these
processes.
[0010] In view of the forgoing, one objective of the present
invention is to provide an aerosol assisted chemical vapor
deposition (AACVD) protocol, which uses solution-based precursors
with a deposition step on a pre-heated substrate. During deposition
via AACVD, the particle growth and sintering processes
simultaneously occur on the substrate surface to develop well
interconnected morphological features and produce adhesive film
electrodes in a matter of minutes. This was used to generate films
of Co--V mixed oxide for effective and stable water oxidation.
BRIEF SUMMARY OF THE INVENTION
[0011] According to a first aspect, the present disclosure relates
to a composite thin film electrode, which comprises a CoVO.sub.x
layer having an average thickness of 500 nm-5 .mu.m in contact with
a substrate, the CoVO.sub.x layer comprising amorphous CoVO.sub.x
having a Co:V molar ratio in a range of 1.0:1.2-1.5:1.0.
[0012] In one embodiment, the CoVO.sub.x layer is porous with a
pore size in a range of 2-200 nm.
[0013] In one embodiment, the composite thin film electrode has an
electrochemically active surface area in a range of 12-22
mF/cm.sup.2.
[0014] In one embodiment, the CoVO.sub.x layer consists essentially
of amorphous CoVO.sub.x.
[0015] In one embodiment, the CoVO.sub.x layer has an O:Co molar
ratio in a range of 4:1 to 9:1.
[0016] In one embodiment, the substrate is a transparent conducting
film selected from the group consisting of FTO, ITO, AZO, GZO, IZO,
IZTO, IAZO, IGZO, IGTO, and ATO.
[0017] According to a second aspect, the present disclosure relates
to a method of making the composite thin film electrode of the
first aspect. The method involves contacting an aerosol with the
substrate to deposit the CoVO.sub.x layer to form the composite
thin film electrode. The aerosol comprises a carrier gas, and a
cobalt complex and a vanadium complex dissolved in a solvent. The
substrate has a temperature in a range of 425-525.degree. C. during
the contacting.
[0018] In one embodiment, the cobalt complex and the vanadium
complex each independently comprise at least one ligand selected
from the group consisting of acetylacetonate, acetate ligand,
trifluoroacetate, isopropanol, and tetrahydrofuran.
[0019] In one embodiment, the cobalt complex is Co(II)
acetylacetonate, and the vanadium complex is V(III)
acetylacetonate.
[0020] In one embodiment, before the contacting, the aerosol
consists essentially of the carrier gas, the cobalt complex, the
vanadium complex, and the solvent.
[0021] In one embodiment, a weight ratio of the cobalt complex to
the solvent in the aerosol, and/or a weight ratio of the vanadium
complex to the solvent in the aerosol is in a range of
1:1,000-1:5.
[0022] In one embodiment, the aerosol is contacted with the
substrate for a time period of 10-30 min.
[0023] In one embodiment, the carrier gas has a flow rate in a
range of 20-250 cm.sup.3/min during the contacting.
[0024] According to a third aspect, the present disclosure relates
to an electrochemical cell comprising the composite thin film
electrode of the first aspect, a counter electrode, and
[0025] an electrolyte solution in contact with both electrodes.
[0026] In one embodiment, the composite thin film electrode has an
overpotential in a range of 270-335 mV at a current density of 9-11
mA/cm.sup.2.
[0027] In one embodiment, the composite thin film electrode has a
current density of 1.0-10.0 mA/cm.sup.2 when the electrodes are
subjected to a bias potential of 1.45-1.55 V.
[0028] In one embodiment, the electrolyte solution comprises water
and an inorganic base having a concentration of 0.1-1.0 M.
[0029] In one embodiment, the composite thin film electrode has a
mass activity in range of 38-50 A/g at 350 mV.
[0030] According to a fourth aspect, the present disclosure relates
to a method for decomposing water into H.sub.2 and O.sub.2. This
involves subjecting the electrodes of the electrochemical cell of
claim 14 with a potential of 0.5-2.0 V.
[0031] In one embodiment, the method also involves separately
collecting H.sub.2-enriched gas and O.sub.2-enriched gas.
[0032] The foregoing paragraphs have been provided by way of
general introduction, and are not intended to limit the scope of
the following claims. The described embodiments, together with
further advantages, will be best understood by reference to the
following detailed description taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] A more complete appreciation of the disclosure and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0034] FIG. 1 is a volcano plot describing mass activity of oxygen
evolution reactions against M-OH bond strength.
[0035] FIG. 2A is a large area, low resolution (10K.times.) FESEM
of the CoVO.sub.x-20 film.
[0036] FIG. 2B is a high resolution (50K.times.) FESEM of the
CoVO.sub.x-20 film.
[0037] FIG. 2C is a cross-section FESEM of the CoVO.sub.x-20 film
on a FTO glass substrate.
[0038] FIG. 2D is a large area, low resolution (10K.times.) FESEM
of the CoVO.sub.x-40 film.
[0039] FIG. 2E is a high resolution (50K.times.) FESEM of the
CoVO.sub.x-40 film.
[0040] FIG. 2F is a cross-section FESEM of the CoVO.sub.x-40 film
on a FTO glass substrate.
[0041] FIG. 2G is a large area, low resolution (10K.times.) FESEM
of the CoVO.sub.x-60 film.
[0042] FIG. 2H is a high resolution (50K.times.) FESEM of the
CoVO.sub.x-60 film.
[0043] FIG. 2I is a cross-section FESEM of the CoVO.sub.x-60 film
on a FTO glass substrate.
[0044] FIG. 3A is EDX spectra of the CoVO.sub.x-20 film.
[0045] FIG. 3B is the atomicity recorded of the CoVO.sub.x-20 film
of FIG. 3A.
[0046] FIG. 3C is an FESEM image of the analysis area of the
CoVO.sub.x-20 film used for FIG. 3A.
[0047] FIG. 3D is EDX spectra of the CoVO.sub.x-40 film.
[0048] FIG. 3E is the atomicity recorded of the CoVO.sub.x-40 film
of FIG. 3D.
[0049] FIG. 3F is an FESEM image of the analysis area of the
CoVO.sub.x-40 film used for FIG. 3A.
[0050] FIG. 3G is EDX spectra of the CoVO.sub.x-60 film.
[0051] FIG. 3H is the atomicity recorded of the CoVO.sub.x-60 film
of FIG. 3G.
[0052] FIG. 3I is an FESEM image of the analysis area of the
CoVO.sub.x-60 film used for FIG. 3G.
[0053] FIG. 4A is a high resolution XPS of the CoVO.sub.x-20 film
showing the binding energies for Co 2p.
[0054] FIG. 4B is a high resolution XPS of the CoVO.sub.x-20 film
showing the binding energies for V 2p.
[0055] FIG. 4C is a high resolution XPS of the CoVO.sub.x-20 film
showing the binding energies for O 1s.
[0056] FIG. 5A is a graph of the forward potential sweeps for
fabricated mixed oxide electrocatalytic materials in 0.5 M KOH
electrolyte solution and at the scan rate of 10 mV s.sup.-1.
[0057] FIG. 5B is a graph of the overpotential values of the same
films of FIG. 5A at a current density of 10 mA cm.sup.-2.
[0058] FIG. 6A shows LSV curves for the CoVO.sub.x-20 film at
different scan rates.
[0059] FIG. 6B shows a zoomed in LSV curve for the water oxidation
reaction at a scan rate of 1 mV sec.sup.-1.
[0060] FIG. 7 is a graph illustrating the Tafel plots for different
films at a scan rate of 10 mV/sec as well as CoVO.sub.x-20 film at
a scan rate of 1.0 mV/sec in 0.5 M KOH electrolyte solution.
[0061] FIG. 8A shows the long term stability tests of the prepared
films at constant current density of 20 mA/cm.sup.2 for more than 5
h and a constant current density of 100 mA/cm.sup.2 for more than 5
h.
[0062] FIG. 8B shows the first and the 500th scan of LSV
measurement with no significant changes in overvoltage and current
density.
[0063] FIG. 9 is a plausible reaction mechanism for electroxidation
of water in the absence and presence of vanadium in a cobalt
electrocatalyst.
[0064] FIG. 10 is a schematic of the AACVD setup used for the
synthesis of the CoVO.sub.x films.
[0065] FIG. 11 shows XRD patterns of Co--V mixed oxide film
electrodes, CoVO.sub.x-20, CoVO.sub.x-40, and CoVO.sub.x-60
fabricated in 20, 40, and 60 min of deposition time at 475.degree.
C.
[0066] FIG. 12A shows XRD patterns from films produced using the
Co(acac).sub.2 precursor and without the V(acac).sub.3
precursor.
[0067] FIG. 12B shows XRD patterns from films produced using the
V(acac).sub.3 precursor and without the Co(acac).sub.2
precursor.
[0068] FIG. 13A shows an FESEM image of the CoVO.sub.x-20 film.
[0069] FIG. 13B is an EDX mapping of VKal obtained from the region
shown in FIG. 13A.
[0070] FIG. 13C is an EDX mapping of CoKal obtained from the region
shown in FIG. 13A.
[0071] FIG. 13D shows an FESEM image of the CoVO.sub.x-40 film.
[0072] FIG. 13E is an EDX mapping of VKal obtained from the region
shown in FIG. 13D.
[0073] FIG. 13F is an EDX mapping of CoKal obtained from the region
shown in FIG. 13D.
[0074] FIG. 13G shows an FESEM image of the CoVO.sub.x-60 film.
[0075] FIG. 13H is an EDX mapping of VKal obtained from the region
shown in FIG. 13G.
[0076] FIG. 13I is an EDX mapping of CoKal obtained from the region
shown in FIG. 13G.
[0077] FIG. 14 shows the current density vs. the scan rate for
experiments in the non-faradaic zone, and the calculated slope
values for the three different films.
[0078] FIG. 15 shows a Nyquist plot for CoVO.sub.x-20,
CoVO.sub.x-40, and CoVO.sub.x-60 films at an applied potential of
1.48 V vs. RHE in the frequency range of 0.1 Hz to 100 KHz.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0079] Embodiments of the present disclosure will now be described
more fully hereinafter with reference to the accompanying drawings,
in which some, but not all embodiments of the disclosure are
shown.
[0080] The present disclosure will be better understood with
reference to the following definitions. As used herein, the words
"a" and "an" and the like carry the meaning of "one or more."
Within the description of this disclosure, where a numerical limit
or range is stated, the endpoints are included unless stated
otherwise. It will be further understood that the terms "comprises"
and/or "comprising," when used in this specification, specify the
presence of stated features, integers, steps, operations, elements,
and/or components, but do not preclude the presence or addition of
one or more other features, integers, steps, operations, elements,
components, and/or groups thereof.
[0081] As used herein, the words "about," "approximately," or
"substantially similar" may be used when describing magnitude
and/or position to indicate that the value and/or position
described is within a reasonable expected range of values and/or
positions. For example, a numeric value may have a value that is
+/-0.1% of the stated value (or range of values), +/-1% of the
stated value (or range of values), +/-2% of the stated value (or
range of values), +/-5% of the stated value (or range of values),
+/-10% of the stated value (or range of values), +/-15% of the
stated value (or range of values), or +/-20% of the stated value
(or range of values). Within the description of this disclosure,
where a numerical limit or range is stated, the endpoints are
included unless stated otherwise. Also, all values and subranges
within a numerical limit or range are specifically included as if
explicitly written out.
[0082] As used herein, "compound" is intended to refer to a
chemical entity, whether as a solid, liquid, or gas, and whether in
a crude mixture or isolated and purified.
[0083] As used herein, "composite" refers to a combination of two
or more distinct constituent materials into one. The individual
components, on an atomic level, remain separate and distinct within
the finished structure. The materials may have different physical
or chemical properties, that when combined, produce a material with
characteristics different from the original components. In some
embodiments, a composite may have at least two constituent
materials that comprise the same empirical formula but are
distinguished by different densities, crystal phases, or a lack of
a crystal phase (i.e. an amorphous phase).
[0084] The present disclosure is intended to include all hydration
states of a given compound or formula, unless otherwise noted or
when heating a material. For example, Ni(NO.sub.3).sub.2 includes
anhydrous Ni(NO.sub.3).sub.2, Ni(NO.sub.3).sub.2.6H.sub.2O, and any
other hydrated forms or mixtures. CuCl.sub.2 includes both
anhydrous CuCl.sub.2 and CuCl.sub.2.2H.sub.2O.
[0085] In addition, the present disclosure is intended to include
all isotopes of atoms occurring in the present compounds and
complexes. Isotopes include those atoms having the same atomic
number but different mass numbers. By way of general example, and
without limitation, isotopes of hydrogen include deuterium and
tritium. Isotopes of carbon include .sup.13C and .sup.14C. Isotopes
of nitrogen include .sup.14N and .sup.15N. Isotopes of oxygen
include .sup.16O, .sup.17O, and .sup.18O. Isotopes of cobalt
include .sup.56Co, .sup.57Co, .sup.58Co, .sup.59Co, and .sup.60Co.
Isotopes of vanadium include .sup.48V, .sup.49V, .sup.50V, and
.sup.51V. Isotopically-labeled compounds of the disclosure may
generally be prepared by conventional techniques known to those
skilled in the art or by processes analogous to those described
herein, using an appropriate isotopically-labeled reagent in place
of the non-labeled reagent otherwise employed.
[0086] As defined here, an aerosol is a suspension of solid or
liquid particles in a gas. An aerosol includes both the particles
and the suspending gas. Primary aerosols contain particles
introduced directly into the gas, while secondary aerosols form
through gas-to-particle conversion. There are several measures of
aerosol concentration. Environmental science and health fields
often use the mass concentration (M), defined as the mass of
particulate matter per unit volume with units such as
.mu.g/m.sup.3. Also commonly used is the number concentration (N),
the number of particles per unit volume with units such as
number/m.sup.3 or number/cm.sup.3. The size of particles has a
major influence on their properties, and the aerosol particle
radius or diameter (d.sub.p) is a key property used to characterize
aerosols. Aerosols vary in their dispersity. A monodisperse
aerosol, producible in the laboratory, contains particles of
uniform size. Most aerosols, however, as polydisperse colloidal
systems, exhibit a range of particle sizes. Liquid droplets are
almost always nearly spherical, but scientists use an equivalent
diameter to characterize the properties of various shapes of solid
particles, some very irregular. The equivalent diameter is the
diameter of a spherical particle with the same value of some
physical property as the irregular particle. The equivalent volume
diameter (d.sub.e) is defined as the diameter of a sphere of the
same volume as that of the irregular particle. Also commonly used
is the aerodynamic diameter. The aerodynamic diameter of an
irregular particle is defined as the diameter of the spherical
particle with a density of 1000 kg/m.sup.3 and the same settling
velocity as the irregular particle.
[0087] As defined here, an electrode is an electrically conductive
material comprising a metal and is used to establish electrical
contact with a nonmetallic part of a circuit. An
"electrically-conductive material" as defined here is a substance
with an electrical resistivity of at most 10.sup.-6 .OMEGA.m,
preferably at most 10.sup.-7 .OMEGA.m, more preferably at most
10.sup.-8 .OMEGA.m at a temperature of 20-25.degree. C. The
electrically-conductive material comprise platinum-iridium alloy,
iridium, titanium, titanium alloy, stainless steel, gold, cobalt
alloy, copper, aluminum, tin, iron, and/or some other metal.
[0088] According to a first aspect, the present disclosure relates
to a composite thin film electrode. The composite thin film
electrode comprises a CoVO.sub.x layer having an average thickness
in a range of 500 nm-5 .mu.m, preferably 700 nm-4 .mu.m, more
preferably 800 nm-3 .mu.m, even more preferably 900 nm-2 .mu.m, or
about 1 .mu.m. In one embodiment, the thickness of the layer may
vary from location to location on the electrode by 1-30%,
preferably 5-20%, relative to the average thickness of the layer.
The CoVO.sub.x layer is a mixed metal oxide and may also be called
a cobalt vanadium oxide layer, or a vanadium cobalt oxide
layer.
[0089] In one embodiment, the CoVO.sub.x layer comprises amorphous
CoVO.sub.x. The amount of amorphous CoVO.sub.x may be measured by
X-ray diffraction patterns. In one embodiment, the CoVO.sub.x layer
consists essentially of amorphous CoVO.sub.x, meaning that the
CoVO.sub.x layer comprises at least 99 wt %, preferably 99.9 wt %,
more preferably 99.95 wt % CoVO.sub.x in an amorphous
(non-crystalline) state, relative to a total weight of the
CoVO.sub.x layer.
[0090] While the name "CoVO.sub.x" when read literally implies a
1:1 molar ratio of Co:V, in some embodiments, this molar ratio may
not be 1:1. In one embodiment, the CoVO.sub.x layer comprises or
consists essentially of CoVO.sub.x having a Co:V molar ratio in a
range of 1.0:1.2-1.5:1.0, preferably 1.0:1.2-1.2:1.0 or
1.0:1.2-1.1:1.0, more preferably 1.0:1.18-1.05:1.0, or about
1.0:1.16, or about 1:1. Here, the CoVO.sub.x layer consisting
essentially of CoVO.sub.x means that at least 99 wt %, preferably
at least 99.9 wt %, more preferably at least 99.95 wt %, or about
100 wt % of the CoVO.sub.x layer, relative to a total weight is
either cobalt, vanadium, or oxygen.
[0091] In one embodiment, the CoVO.sub.x layer has an O:Co molar
ratio in a range of 4:1 to 9:1, preferably 5:1-8.5:1, more
preferably 6:1-8:1, or about 7.8:1. In one embodiment, the
CoVO.sub.x layer has an O:V molar ratio in a range of 4:1 to 9:1,
preferably 5:1-8:1, more preferably 6:1-7:1, or about 6.7:1.
[0092] The CoVO.sub.x layer may be in the form of a mesh,
exfoliated surface, and/or blistered surface. FIG. 2B shows one
such embodiment. The CoVO.sub.x layer may have pores or open
spaces. In one embodiment, the CoVO.sub.x layer may have a porosity
in a range of 10-70%, preferably 20-60%. In a related embodiment,
the CoVO.sub.x layer may have a surface area per mass CoVO.sub.x
layer of 80-350 m.sup.2/g, preferably 100-250 m.sup.2/g, even more
preferably 120-220 m.sup.2/g. In an alternative embodiment, the
CoVO.sub.x layer may be in the form of particles, cylinders, boxes,
spikes, flakes, plates, ellipsoids, toroids, stars, ribbons, discs,
rods, granules, prisms, cones, flakes, platelets, sheets, or some
other shape.
[0093] In one embodiment, the CoVO.sub.x layer is porous with a
pore size in a range of 2-200 nm, preferably 2-100 nm, more
preferably 3-50 nm, even more preferably 3-10 nm, or 2-10 nm, or
2-5 nm. In one embodiment, the CoVO.sub.x layer is monolithic,
meaning that all parts of the layer are attached to one another as
a single structure, as opposed to the layer being fragmented or in
the form of particles. In one embodiment, the CoVO.sub.x layer is
in the form of a mesh having strands or wires of CoVO.sub.x with
diameters in a range of 100-250 nm, 120-220 nm, the strands or
wires being joined together. FIG. 2B shows one such example.
[0094] In one embodiment, the pores of the CoVO.sub.x layer are
monodisperse in diameter, having a coefficient of variation or
relative standard deviation, expressed as a percentage and defined
as the ratio of the pore diameter standard deviation (.sigma.) to
the pore diameter mean (.mu.), multiplied by 100%, of less than
25%, preferably less than 10%, preferably less than 8%, preferably
less than 6%, preferably less than 5%. In a preferred embodiment,
the pores are monodisperse having a pore diameter distribution
ranging from 80% of the average pore diameter to 120% of the
average pore diameter, preferably 85-115%, preferably 90-110% of
the average pore diameter. In another embodiment, the pore
diameters are not monodisperse.
[0095] In one embodiment, the CoVO.sub.x layer is in contact with a
substrate. In one embodiment, the substrate is a transparent
conducting film selected from the group consisting of FTO
(fluorine-doped tin oxide), ITO (indium tin oxide), AZO
(aluminum-doped zinc oxide), GZO (gallium-doped zinc oxide), IZO
(indium zinc oxide), IZTO (indium zinc tin oxide), IAZO (indium
aluminum zinc oxide), IGZO (indium gallium zinc oxide), IGTO
(indium gallium tin oxide), and ATO (antimony tin oxide). In other
embodiments, transparent conducting polymers (such as PEDOT) or
carbon nanotubes may be used with or in place of the compounds
previously mentioned. In a preferred embodiment, the substrate is
FTO. The transparent conducting film may have an average thickness
of 1 .mu.m-1 mm, preferably 10 .mu.m-900 .mu.m, more preferably 200
.mu.m-800 .mu.m, or about 600 .mu.m. Alternatively, the transparent
conducting film may have an average thickness of 500 nm-200 .mu.m,
preferably 1 .mu.m-100 .mu.m, more preferably 10 .mu.m-50 .mu.m.
However, in some embodiments, the transparent conducting film may
have an average thickness of less than 500 nm. For instance, the
transparent conducting film may have an average thickness of 50-500
nm, 80-300 nm, or 100-250 nm. Preferably the transparent conducting
film is attached to an additional support, such as a glass slide.
However, in other embodiments, the substrate may be glass, quartz,
ceramic, a metal, a composite material, or a polymeric material
having temperature resistance at least up to the temperature of the
substrate heating. Where the substrate comprises glass, the glass
may be boro-aluminosilicate glass, sodium borosilicate glass,
fused-silica glass, soda lime glass, or some other type of
glass.
[0096] In a preferred embodiment, the substrate is substantially
flat, smooth, and planar. Here, the substrate may have a thickness
at any and every point on the substrate that varies by less than 4
nm, preferably by less than 3 nm, more preferably by less than 2
nm, even more preferably by less than 1 nm, less than 0.5 nm, or
less than 0.3 nm than the average thickness. In one embodiment, the
substrate is smooth and without pores, nanostructures, or
microstructures. In a related embodiment, the substrate is
rectangular and essentially smooth so that the substrate and a
three dimensional convex hull of the substrate occupy essentially
the same volume. In another related embodiment, the bulk volume of
the substrate is the same as its actual volume, its actual volume
being measured by fluid displacement or some other method.
[0097] In one embodiment, the substrate has a sheet resistance in a
range of 1-40 .OMEGA.sq.sup.-1, preferably 2-20 .OMEGA.sq.sup.-1,
more preferably 4-12 .OMEGA.sq.sup.-1, or about 8 .OMEGA.sq.sup.-1.
Preferably, the CoVO.sub.x layer in contact with the substrate
forms an electrically-conductive material with the transparent
conducting film. An "electrically-conductive material" as defined
here is a substance with an electrical resistivity of at most
10.sup.-6 .OMEGA.m, preferably at most 10.sup.-7 .OMEGA.m, more
preferably at most 10.sup.-8 .OMEGA.m at a temperature of
20-25.degree. C.
[0098] In one embodiment, the composite thin film electrode has an
electrochemically active surface area (ECSA, or electroactive
surface area) in a range of 12-22 mF/cm.sup.2, preferably 13-20
mF/cm.sup.2, more preferably 14-19 mF/cm.sup.2, or about 17.6
mF/cm.sup.2.
[0099] In one embodiment, the composite thin film electrode does
not comprise a nickel foam, a gold foam, or some other metallic
foam. In another embodiment, the composite thin film electrode does
not comprise carbon. In one embodiment, the composite thin film
electrode consists essentially of Co, V, O, and FTO coated
glass.
[0100] According to a second aspect, the present disclosure relates
to a method of making the composite thin film electrode of the
first aspect. This method involves contacting an aerosol with a
substrate to deposit the CoVO.sub.x layer to form the composite
thin film electrode. As described here, "contacting an aerosol with
a substrate" is considered to be synonymous with "contacting a
substrate with an aerosol." Both phrases mean that the substrate is
exposed to the aerosol, so that a portion of the suspended
particles of the aerosol directly contact the substrate. Contacting
may also be considered equivalent to "introducing" or "depositing,"
such as "depositing an aerosol on a substrate." In one embodiment,
the contacting may be considered aerosol-assisted chemical vapor
deposition (AACVD). In one embodiment, the method of making the
composite thin film electrode may be considered a one-step method,
as the formation of the CoVO.sub.x layer takes place immediately
following and/or during the contacting of the aerosol with the
substrate.
[0101] In one embodiment, the temperature of the substrate during
the contacting is in a range of 425-550.degree. C., preferably
450-525.degree. C., more preferably 460-500.degree. C., even more
preferably 465-480.degree. C., or about 475.degree. C. In one
embodiment, the temperature of the substrate during the contacting
never reaches a temperature of greater than 550.degree. C.,
preferably no greater than 500.degree. C., more preferably no
greater than 480.degree. C.
[0102] The aerosol comprises a carrier gas, a cobalt complex, a
vanadium complex, and a solvent. In one embodiment, the aerosol
consists of, or consists essentially of, a carrier gas, a cobalt
complex, a vanadium complex, and a solvent before the contacting,
preferably immediately before the contacting. Preferably, the
cobalt complex and vanadium complex are dissolved or dispersed in
the solvent. In some embodiments, the cobalt complex and vanadium
complex are dissolved in the same aerosol droplets. In other
embodiments, some aerosol droplets may consist of the cobalt
complex and solvent, and other aerosol droplets may consist of
vanadium complex and solvent. Similarly, some aerosol droplets may
consist of only solvent.
[0103] In one embodiment, the cobalt complex has an acetylacetone
or acetylacetonate (acac) ligand, a trifluoroacetate (TFA) ligand,
an acetate ligand (OAc), an isopropanol (.sup.iPrOH) ligand, a
tetrahydrofuran (THF) ligand, and/or a water (H.sub.2O) ligand. In
one embodiment, a molar ratio of acetylacetonate ligands to Co in
the cobalt complex is in a range of 1:1-3:1, or about 2:1. In one
embodiment, the cobalt complex is Co(II) acetylacetonate, or
Co(acac).sub.2. In alternative embodiments, the cobalt complex may
be bromopentaamminecobalt(III) bromide, caesium
hexafluorocobaltate(IV), chloro(pyridine)cobaloxime,
chloropentamminecobalt chloride,
cis-dichlorobis(ethylenediamine)cobalt(III) chloride,
trans-dichlorobis(ethylenediamine)cobalt(III) chloride,
hexamminecobalt(III) chloride, nitropentaamminecobalt(III)
chloride, tetracobalt dodecacarbonyl,
tris(ethylenediamine)cobalt(III) chloride or some other cobalt
complex or cobalt salt. In these alternative embodiments, the
cobalt may have a II, III, or IV oxidation state.
[0104] In one embodiment, the vanadium complex has an acetylacetone
or acetylacetonate (acac) ligand, a trifluoroacetate (TFA) ligand,
an acetate ligand (OAc), an isopropanol (.sup.iPrOH) ligand, a
tetrahydrofuran (THF) ligand, and/or a water (H.sub.2O) ligand. In
one embodiment, the vanadium complex is V(III) acetylacetonate, or
V(acac).sub.2. In one embodiment, a molar ratio of acetylacetone
ligands to V in the vanadium complex is in a range of 1:1-4:1,
2:1-4:1, or about 3:1. In alternative embodiments, without
limitation, the vanadium complex may be vanadium acetylacetonate,
vanadium hexacarbonyl, vanadocene, vanadyl perchlorate, vanadyl
acetylacetonate, ammonium metavanadate, vanadocene dichloride, or
some other vanadium complex or vanadium salt, such as a vanadium
halide. In these alternative embodiments, the vanadium may have a
II, III, IV, or V oxidation state.
[0105] In alternative embodiments, the cobalt complex and/or the
vanadium complex may not comprise one or more of the ligands
acetylacetonate, trifluoroacetate, acetate, isopropanol,
tetrahydrofuran, or water, and in other embodiments, one or more
ligands may be substituted with other ligands, such as ethanol. In
one embodiment, other ligands may be present in the cobalt complex
and/or the vanadium complex, including but not limited to
acetonitrile, methyl isocyanide, phosphine, bipyridine,
nitrilotriacetic acid, and diimine.
[0106] In one embodiment, the cobalt complex and the solvent are
present in the aerosol at a cobalt complex to solvent weight ratio
of 1:1,000-1:5, preferably 1:800-1:50, more preferably 1:600-1:70,
even more preferably 1:500-1:100, or about 1:365.
[0107] In one embodiment, the vanadium complex and the solvent are
present in the aerosol at a vanadium complex to solvent weight
ratio of 1:1,000-1:5, preferably 1:800-1:50, more preferably
1:600-1:70, even more preferably 1:500-1:100, or about 1:322.
[0108] In one embodiment, the cobalt complex and the vanadium
complex are present in the aerosol at a cobalt to vanadium molar
ratio of 1.0:1.2-1.5:1.0, preferably 1.0:1.2-1.4:1.0, or
1.0:1.2-1.2:1.0, or 1.0:1.2-1.1:1.0, more preferably
1.0:1.18-1.05:1.0, or about 1.0:1.16, or about 1.36:1.0, or about
1.16:1.0, or about 1:1.
[0109] In an alternative embodiment, rather than a cobalt complex
and a vanadium complex existing as separate molecules, a single
molecule comprising both vanadium and cobalt may be used in the
aerosol.
[0110] In one embodiment, the carrier gas is N.sub.2, He,
compressed air, and/or Ar. Preferably the carrier gas is
N.sub.2.
[0111] In one embodiment, the solvent may be toluene,
tetrahydrofuran, acetic acid, acetone, acetonitrile, butanol,
dichloromethane, chloroform, chlorobenzene, dichloroethane,
diethylene glycol, diethyl ether, dimethoxy-ethane,
dimethyl-formamide, dimethyl sulfoxide, ethanol, ethyl acetate,
ethylene glycol, heptane, hexamethylphosphoramide,
hexamethylphosphorous triamide, methanol, methyl t-butyl ether,
methylene chloride, pentane, cyclopentane, hexane, cyclohexane,
benzene, dioxane, propanol, isopropyl alcohol, pyridine, triethyl
amine, propandiol-1,2-carbonate, ethylene carbonate, propylene
carbonate, nitrobenzene, formamide, .gamma.-butyrolactone, benzyl
alcohol, n-methyl-2-pyrrolidone, acetophenone, benzonitrile,
valeronitrile, 3-methoxy propionitrile, dimethyl sulfate, aniline,
n-methylformamide, phenol, 1,2-dichlorobenzene, tri-n-butyl
phosphate, ethylene sulfate, benzenethiol, dimethyl acetamide,
N,N-dimethylethaneamide, 3-methoxypropionnitrile, diglyme,
cyclohexanol, bromobenzene, cyclohexanone, anisole,
diethylformamide, 1-hexanethiol, ethyl chloroacetate,
1-dodecanthiol, di-n-butylether, dibutyl ether, acetic anhydride,
m-xylene, o-xylene, p-xylene, morpholine, diisopropyl etheramine,
diethyl carbonate, 1-pentandiol, n-butyl acetate, and/or
1-hexadecanthiol. In one embodiment, the solvent comprises
pyridine, N,N-dimethylformamide (DMF), N,N-dimethylacetamide,
N-methyl pyrrolidone (NMP), hexamethylphosphoramide (HMPA),
dimethyl sulfoxide (DMSO), acetonitrile, tetrahydrofuran (THF),
1,4-dioxane, dichloromethane, chloroform, carbon tetrachloride,
dichloroethane, acetone, ethyl acetate, pentane, hexane, decalin,
dioxane, benzene, toluene, xylene, o-dichlorobenzene, diethyl
ether, methyl t-butyl ether, methanol, ethanol, ethylene glycol,
isopropanol, propanol, and/or n-butanol. In a preferred embodiment,
the solvent is acetone, methanol, ethanol, and/or isopropanol. More
preferably the solvent is methanol, and in another embodiment, the
solvent consists essentially of methanol.
[0112] In one embodiment, the solvent may comprise water. The water
used as a solvent or for other purposes may be tap water, distilled
water, bidistilled water, deionized water, deionized distilled
water, reverse osmosis water, and/or some other water. In one
embodiment the water is bidistilled or treated with reverse osmosis
to eliminate trace metals. Preferably the water is bidistilled,
deionized, deionized distilled, or reverse osmosis water, and at
25.degree. C. has a conductivity of less than 10 .mu.Scm.sup.-1,
preferably less than 1 .mu.Scm.sup.-1; a resistivity of greater
than 0.1 M.OMEGA.cm, preferably greater than 1 M.OMEGA.cm, more
preferably greater than 10 M.OMEGA.cm; a total solid concentration
of less than 5 mg/kg, preferably less than 1 mg/kg; and a total
organic carbon concentration of less than 1000 .mu.g/L, preferably
less than 200 .mu.g/L, more preferably less than 50 .mu.g/L.
[0113] Preferably the solvent and the cobalt complex and/or
vanadium complex are able to form an appropriately soluble solution
that can be dispersed in the carrier gas as aerosol particles. For
instance, the cobalt complex and/or vanadium complex may first be
dissolved in a volume of solvent, and then pumped through a jet
nozzle in order to create an aerosol mist. In other embodiments,
the mist may be generated by a piezoelectric ultrasonic generator.
Other nebulizers and nebulizer arrangements may also be used, such
as concentric nebulizers, cross-flow nebulizers, entrained
nebulizers, V-groove nebulizers, parallel path nebulizers, enhanced
parallel path nebulizers, flow blurring nebulizers, and
piezoelectric vibrating mesh nebulizers. In one embodiment, the
mixtures of the cobalt complex and solvent, and the vanadium
complex and solvent, are introduced as separate aerosols, for
instance, produced by seprate nozzles or nebulizers. Preferably,
however, the cobalt complex and vanadium complex are mixed together
in the same solvent prior to producing the aerosol.
[0114] In one embodiment, the aerosol may have a mass concentration
M, of 10 .mu.g/m.sup.3-1,000 mg/m.sup.3, preferably 50
.mu.g/m.sup.3-1,000 .mu.g/m.sup.3. In one embodiment, the aerosol
may have a number concentration N, in a range of 10.sup.3-10.sup.6,
preferably 10.sup.4-10.sup.5 cm.sup.-3. In other embodiments, the
aerosol may have a number concentration of less than 10.sup.3 or
greater than 10.sup.6. The aerosol particles or droplets may have
an equivalent volume diameter (d.sub.e) in a range of 20 nm-100
.mu.m, preferably 0.5-70 .mu.m, more preferably 1-50 .mu.m, though
in some embodiments, aerosol particles or droplets may have an
average diameter of smaller than 0.2 .mu.m or larger than 100
.mu.m.
[0115] In an alternative embodiment, the oxidation state of the
vanadium in the vanadium complex, and/or the cobalt in the cobalt
complex, may be reduced or oxidized in the process of the
deposition and formation of the CoVO.sub.x layer. In one
embodiment, the aerosol and substrate do not comprise or contact
hydrogen gas or a reducing agent during the contacting and/or
depositing. In a related embodiment, the aerosol and substrate do
not comprise or contact hydrogen gas or a reducing agent
immediately prior to the contacting and/or depositing. In one
embodiment, the reaction chamber where the depositing takes place
is essentially free of hydrogen gas and a reducing agent
immediately prior to the contacting. In one embodiment, an
intermediate reducing agent is created during the contacting.
[0116] In a related embodiment, before the contacting and/or
depositing, the aerosol consists essentially of the carrier gas,
the solvent, the vanadium complex, and the cobalt complex, meaning
that at least 99.9 wt %, preferably at least 99.99 wt %, or 100 wt
% of the aerosol is carrier gas, solvent, vanadium complex, or
cobalt complex, relative to a total weight of the aerosol.
[0117] In one embodiment, the aerosol is contacted with the
substrate for a time period of 10-30 min, preferably 12-28 min,
more preferably 15-25 min, even more preferably 17-23 min, or about
20 min.
[0118] In one embodiment, during the contacting of the aerosol, the
carrier gas has a flow rate in a range of 20-250 cm.sup.3/min,
preferably 50-230 cm.sup.3/min, more preferably 75-200
cm.sup.3/min, even more preferably 100-150 cm.sup.3/min, or about
120 cm.sup.3/min. Preferably, the aerosol has a flow rate similar
to the carrier gas, with the exception of the portion of aerosol
that gets deposited on the substrate. In one embodiment, the
aerosol may enter the chamber and the flow rate may be stopped, so
that a portion of aerosol may settle on the substrate.
[0119] In one embodiment, the aerosol is contacted with the
substrate in a reaction chamber. The flow of the carrier gas and
aerosol may have a gas hourly space velocity in a range of 10-1,000
h.sup.-1, preferably 50-500 h.sup.-1, more preferably 100-130
h.sup.-1.
[0120] The contacting and/or introducing may take place within a
closed chamber or reactor, and the aerosol may be generated by
dispersing a solution of the cobalt complex and/or vanadium complex
and solvent. In one embodiment, this dispersing may be increased by
fans, jets, or pumps. However, in another embodiment, an aerosol
may be formed in a closed chamber with a substrate where the
aerosol particles are allowed to diffuse towards or settle on the
substrate. The substrate may have an area in a range of 0.5-4
cm.sup.2, preferably 1.0-3 cm.sup.2, more preferably 1.5-3
cm.sup.2. In one embodiment, the closed chamber or reactor may have
a length of 10-100 cm, preferably 12-30 cm, and a diameter or width
of 1-10 cm, preferably 2-5 cm. In other embodiments, the closed
chamber or reactor may have an interior volume of 0.2-100 L,
preferably 0.3-25 L, more preferably 0.5-10 L. In one embodiment,
the closed chamber or reactor may comprise a cylindrical glass
vessel, such as a glass tube.
[0121] Being in a closed chamber, the interior pressure of the
chamber (and thus the pressure of the aerosol) may be controlled.
The pressure may be practically unlimited, but need not be an
underpressure or an overpressure. Preferably the chamber and
substrate are heated and held at a temperature prior to the
contacting, so that the temperature may stabilize. The chamber and
substrate may be heated for a time period of 5 min-1 hr, preferably
10-20 min prior to the contacting.
[0122] During the contacting of the aerosol, the CoVO.sub.x layer
may form at a rate of 0.1 to 20, 0.2 to 18, 0.4 to 16, 0.5 to 14,
0.6 to 12, 0.7 to 10, 0.8 to 9, 3 to 15, 1.0 to 8, 1.5 to 5, or 2
to 3 nm/s, and/or at least 0.01, 0.05, 0.1, 0.2, 0.4, 0.5, 0.6,
0.8, 1.0, 1.5, 1.75, 2, 2.5, 3.33, 3.5, 4, 4.5, 5, 6.5, 7, 7.5,
7.75, 8, 8.25, 8.5, 8.75, 9, or 10 nm/s. In one embodiment, the
layer may form at a rate in a range of 0.1-2.0 nm/s, 0.2-1.9 nm/s,
0.3-1.8 nm/s, 0.4-1.7 nm/s, 0.5-1.6 nm/s, 0.6-1.5 nm/s, 0.7-1.3
nm/s, or about 0.8 nm/s, or about 1.1 nm/s, or about 1.3 nm/s.
[0123] In one embodiment, the method of making the composite thin
film electrode may further comprise a step of cooling the composite
thin film electrode after the contacting. The composite thin film
electrode may be cooled to a temperature between 10 to 45.degree.
C., 20 to 40.degree. C., or 25 to 35.degree. C. under an inert gas
(such as N.sub.2 or Ar) over a time period of 0.5 to 5 h, 0.75 to 4
h, 1 to 3 h, 1.25 to 2.5 h, or 1.5 to 2 h. In one embodiment, the
composite thin film electrode may be left in the chamber and
allowed to cool.
[0124] In one embodiment, the method of making the composite thin
film electrode may further comprise a step of preparing the cobalt
complex before the contacting. The cobalt complex may be
synthesized by methods described herein, or by mixing
Co(OAc).sub.2, Ti(Pro).sub.4, and trifluoroacetic acid in THF to
form a mixture. The mixture may be stirred for 0.5-6 h, preferably
1-3 h under an inert atmosphere of N.sub.2 or Ar gas. The reaction
mixture may then be dried to yield the cobalt complex, or
alternatively, the reaction mixture may be dried, resuspended in
THF, and then dried a second time to yield the cobalt complex.
[0125] An example AACVD setup is illustrated in FIG. 10. Here, a
container of the Co--V precursor solution 12 (of solvent, cobalt
complex, and vanadium complex) is connected to a carrier gas supply
10 and placed in an ultrasonic humidifier 14. Aerosol droplets 22
are transferred into a reactor tube 20. The reactor tube 20 is
positioned in a tube furnace 16 with heating zones 18. The aerosol
droplets 22 deposit on substrate slides 24 within the reactor tube
20. To support a flow of aerosol, the reactor tube 20 is also
connected to a gas trap 26 and an exhaust line 28.
[0126] In an alternative embodiment, the composite thin film
electrode may be formed by drop-drying or immobilizing CoVO.sub.x
on a conductive substrate, such as onto an ITO film or a gold film,
or on a carbon substrate. In an alternative embodiment, the
composite thin film electrode, or some other electrode involving
nanostructured CoVO.sub.x, may be formed by lithography, more
preferably nanolithography. Nanolithography techniques may be
categorized as in series or parallel, mask or
maskless/direct-write, top-down or bottom-up, beam or tip-based,
resist-based or resist-less methods all of which are acceptable in
terms of the present disclosure. Exemplary nanolithography
techniques include, but are not limited to, optical lithography,
photolithography, directed self-assembly, extreme ultraviolet
lithography, electron beam lithography, electron beam direct write
lithography, multiple electron beam lithography, nanoimprint
lithography, step-and-flash imprint lithography, multiphoton
lithography, scanning probe lithography, dip-pen nanolithography,
thermochemical nanolithography, thermal scanning probe lithography,
local oxidation nanolithography, molecular self-assembly, stencil
lithography, X-ray lithography, laser printing of single
nanoparticles, magnetolithography, nanosphere lithography, proton
beam writing, charged particle lithography, ion projection
lithography, electron projection lithography, neutral particle
lithography and mixtures thereof. In another alternative
embodiment, the composite thin film electrode may be formed by a
sol-gel, solvothermal synthesis, or chemical vapor deposition
method. In another alternative embodiment, the composite thin film
electrode may be synthesized by two or more techniques, for
instance, a nanolithography method and then an electrodeposition
method.
[0127] In another alternative embodiment, a layer of CoVO.sub.x may
be formed as an electrode, and then etched to form a nanostructured
surface having an increased surface area appropriate for
electrocatalysis.
[0128] According to a third aspect, the present disclosure relates
to an electrochemical cell comprising the composite thin film
electrode of the first aspect, a counter electrode, and an
electrolyte solution in contact with both electrodes. As used
herein, the composite thin film electrode may be considered the
working electrode.
[0129] In one embodiment, the electrochemical cell is a vessel
having an internal cavity for holding the electrolyte solution. The
vessel may be cylindrical, cuboid, frustoconical, spherical, or
some other shape. The vessel walls may comprise a material
including, but not limited to, glass, polypropylene, polyvinyl
chloride, polyethylene, and/or polytetrafluoroethylene, and the
vessel walls may have a thickness of 0.1-3 cm, preferably 0.1-2 cm,
more preferably 0.2-1.5 cm. The internal cavity may have a volume
of 2 mL-100 mL, preferably 2.5 mL-50 mL, more preferably 3 mL-20
mL. In another embodiment, for instance, for small scale or
benchtop water oxidation, the internal cavity may have a volume of
100 mL-50 L, preferably 1 L-20 L, more preferably 2 L-10 L. In
another embodiment, for instance, for pilot plant water oxidation,
the internal cavity may have a volume of 50 L-10,000 L, preferably
70 L-1,000 L, more preferably 80 L-2,000 L. In another embodiment,
for instance, for industrial plant-scale water oxidation, the
internal cavity may have a volume of 10,000 L-500,000 L, preferably
20,000 L-400,000 L, more preferably 40,000 L-100,000 L. In one
embodiment, one or more electrochemical cells may be connected to
each other in parallel and/or in series. In another embodiment, the
electrolyte solution may be in contact with more than one working
electrode and/or more than one counter electrode.
[0130] In one embodiment, the counter electrode comprises gold,
platinum, or carbon. In a further embodiment, the counter electrode
comprises platinum. In one embodiment, the counter electrode may be
in the form of a wire, a rod, a cylinder, a tube, a scroll, a
sheet, a piece of foil, a woven mesh, a perforated sheet, or a
brush. The counter electrode may be polished in order to reduce
surface roughness or may be texturized with grooves, channels,
divots, microstructures, or nanostructures.
[0131] In another further embodiment, where the counter electrode
comprises platinum, the counter electrode is in the form of rod,
wire, or a coiled wire. Alternatively, the counter electrode may
comprise some other electrically-conductive material such as
platinum-iridium alloy, iridium, titanium, titanium alloy,
stainless steel, gold, cobalt alloy and/or some other
electrically-conductive material, where an "electrically-conductive
material" as defined here is a substance with an electrical
resistivity of at most 10.sup.-6 .OMEGA.m, preferably at most
10.sup.-7 .OMEGA.m, more preferably at most 10.sup.-8 .OMEGA.m at a
temperature of 20-25.degree. C. In another alternative embodiment,
the working electrode may not comprise FTO, but may comprise any of
the previously mentioned metals.
[0132] In a preferred embodiment, the counter electrode has at
least one outer surface comprising an essentially inert,
electrically conducting chemical substance, such as platinum, gold,
or carbon. In another embodiment, the counter electrode may
comprise solid platinum, gold, or carbon. The form of the counter
electrode may be generally relevant only in that it needs to supply
sufficient current to the electrolyte solution to support the
current required for electrochemical reaction of interest. The
material of the counter electrode should thus be sufficiently inert
to withstand the chemical conditions in the electrolyte solution,
such as acidic or basic pH values, without substantially degrading
during the electrochemical reaction. The counter electrode
preferably should not leach out any chemical substance that
interferes with the electrochemical reaction or might lead to
undesirable contamination of either electrode.
[0133] In a further embodiment, where the counter electrode
comprises platinum, the counter electrode may be in the form of a
mesh. In one embodiment, the counter electrode in the form of a
mesh may have a nominal aperture or pore diameter of 0.05-0.6 mm,
preferably 0.1-0.5 mm, more preferably 0.2-0.4 mm, and/or a wire
diameter of 0.01-0.5 mm, preferably 0.08-0.4 mm, more preferably
0.1-0.3 mm. In other embodiments, the counter electrode may be
considered a gauze with a mesh number of 40-200, preferably 45-150,
more preferably 50-100. In other embodiments, the counter electrode
may be in the form of a perforated sheet or a sponge. In one
embodiment, the counter electrode may be in the form of a mesh with
one or more bulk dimensions (length, width, or thickness) as
previously described for the composite thin film electrode.
[0134] In one embodiment, the counter electrode is in the form of a
rod or wire. The rod or wire may have straight sides and a circular
cross-section, similar to a cylinder. A ratio of the length of the
rod or wire to its width may be 1,500:1-1:1, preferably 500:1-2:1,
more preferably 300:1-3:1, even more preferably 200:1-4:1. The
length of the rod or wire may be 0.5-50 cm, preferably 1-30 cm,
more preferably 3-20 cm, and a long wire may be coiled or bent into
a shape that allows the entire wire to fit into an electrochemical
cell. The diameter of the rod or wire may be 0.5-20 mm, preferably
0.8-8 mm, more preferably 1-3 mm. In one embodiment, the diameter
of the rod or wire may be smaller, for instance, with a diameter in
a range of 0.1-1 mm, preferably 0.2-0.5 mm, or about 0.25 mm. In
some embodiments, a rod may have an elongated cross-section,
similar to a ribbon or strip of metal.
[0135] In one embodiment, the electrolyte solution comprises water
and an inorganic base at a concentration of 0.1-1.0 M, preferably
0.2-0.8 M, more preferably 0.3-0.7 M, or about 0.5 M, though in
some embodiments, the inorganic base may be present at a
concentration of less than 0.1 M or greater than 1.0 M. For long
term electrocatalysis, the organic base may be present at a
concentration in a range of 0.05-0.5 M, preferably 0.08-0.2 M, more
preferably about 0.1 M. The inorganic base may be KOH, LiOH, NaOH,
Be(OH).sub.2, Mg(OH).sub.2, Ca(OH).sub.2, Sr(OH).sub.2,
Ba(OH).sub.2, or some other inorganic base. Preferably the
inorganic base is KOH. In an alternative embodiment, an organic
base may be used, such as sodium acetate. In another alternative
embodiment, an acid may be used instead of a base.
[0136] The water may be tap water, distilled water, bidistilled
water, deionized water, deionized distilled water, reverse osmosis
water, and/or some other water. In one embodiment the water is
bidistilled to eliminate trace metals. Preferably the water is
bidistilled, deionized, deionized distilled, or reverse osmosis
water and at 25.degree. C. has a conductivity at less than 10
.mu.Scm.sup.-1, preferably less than 1 .mu.Scm.sup.-1, a
resistivity greater than 0.1 M.OMEGA.cm, preferably greater than 1
M.OMEGA.cm, more preferably greater than 10 M.OMEGA.cm, a total
solid concentration less than 5 mg/kg, preferably less than 1
mg/kg, and a total organic carbon concentration less than 1000
.mu.g/L, preferably less than 200 .mu.g/L, more preferably less
than 50 .mu.g/L.
[0137] In one embodiment, the composite thin film electrode has a
current density of 1.0-10 mA/cm.sup.2, preferably 1.2-9.8
mA/cm.sup.2, more preferably 2-9 mA/cm.sup.2 when the electrodes
are subjected to a bias potential of 1.45-1.55 V, preferably
1.47-1.53 V.
[0138] In one embodiment, composite thin film electrode has an
overpotential in a range of 270-335 mV, preferably 280-325 mV, more
preferably 290-320 mV, or about 310 mV at a current density of 9-11
mA/cm.sup.2, 9.5-10.5 mA/cm.sup.2.
[0139] Preferably, to maintain uniform concentrations and/or
temperatures of the electrolyte solution, the electrolyte solution
may be stirred or agitated during the step of the subjecting. The
stirring or agitating may be done intermittently or continuously.
This stirring or agitating may be by a magnetic stir bar, a
stirring rod, an impeller, a shaking platform, a pump, a sonicator,
a gas bubbler, or some other device. Preferably the stirring is
done by an impeller or a magnetic stir bar.
[0140] In one embodiment, a composite thin film electrode may have
a higher current density than a bare FTO, where the FTO electrode
has essentially the same structure without the CoVO.sub.x layer.
For example, the bare carbon electrode may comprise bare carbonized
paper, and may be housed in a similar electrode assembly. Here,
over the same range of electrical potential and in similar
electrochemical cells, the composite thin film electrode may have a
current density that is greater by a factor of 3-12, preferably
4-10, than the current density of the bare carbon electrode. This
difference in current densities may lead to the composite thin film
electrode supporting a faster chemical reaction rate in an
electrochemical cell. In one embodiment, a composite thin film
electrode formed from a shorter deposition time may have a greater
current density than another composite thin film electrode formed
with a longer deposition time, as illustrated in FIG. 5A. This may
be due to the increased surface area and increased electroactive
surface area of the composite thin film electrode formed with the
shorter deposition time.
[0141] In one embodiment, the electrochemical cell further
comprises a reference electrode in contact with the electrolyte
solution. A reference electrode is an electrode which has a stable
and well-known electrode potential. The high stability of the
electrode potential is usually reached by employing a redox system
with constant (buffered or saturated) concentrations of each
relevant species of the redox reaction. A reference electrode may
enable a potentiostat to deliver a stable voltage to the working
electrode or the counter electrode. The reference electrode may be
a standard hydrogen electrode (SHE), a normal hydrogen electrode
(NHE), a reversible hydrogen electrode (RHE), a saturated calomel
electrode (SCE), a copper-copper(II) sulfate electrode (CSE), a
silver chloride electrode (Ag/AgCl), a pH-electrode, a
palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), a
mercury-mercurous sulfate electrode, or some other type of
electrode. In a preferred embodiment, a reference electrode is
present and is a silver chloride electrode (Ag/AgCl), while for
long term electrocatalysis, a saturated calomel electrode (Hg/HgO)
was used. However, in some embodiments, the electrochemical cell
does not comprise a third electrode.
[0142] According to a fourth aspect, the present disclosure relates
to a method for decomposing water into H.sub.2 and O.sub.2. This
method involves the step of subjecting the electrodes of the
electrochemical cell of the third aspect with a potential of
0.5-2.0 V, preferably 0.6-1.8 V, more preferably 0.8-1.7 V. Here,
"the electrodes" refers to the composite thin film electrode and
the counter electrode. However, in some embodiments, the electrodes
may be subjected to a potential of less than 0.5 V or greater than
2.0 V.
[0143] Preferably the composite thin film electrode functions as
the anode, receiving a positive potential to oxidize OH.sup.- into
O.sub.2 gas and H.sub.2O, while the counter electrode functions as
the cathode, receiving a negative potential to reduce water into
H.sub.2 gas and OH.sup.-. This is summarized by the following
reactions:
2H.sub.2O.sub.(l)+2e.sup.-.fwdarw.H.sub.2(g)+2OH.sup.-.sub.(aq)
Cathode (reduction):
4OH.sup.-.sub.(aq).fwdarw.O.sub.2(g)+2H.sub.2O.sub.(l)+4e.sup.-
Anode (oxidation):
2H.sub.2O.sub.(l).fwdarw.2H.sub.2(g)+O.sub.2(g) Overall
reaction:
[0144] In another embodiment, the potentials may be switched,
wherein the composite thin film electrode functions as the cathode
and receives a negative potential, and the counter electrode
functions as the anode and receives a positive potential. In an
alternative embodiment, the electrodes may be subjected to an
alternating current (AC) in which the anode and cathode roles are
continually switched between the two electrodes.
[0145] In one embodiment, the potential may be applied to the
electrodes by a battery, such as a battery comprising one or more
electrochemical cells of alkaline, lithium, lithium-ion,
nickel-cadmium, nickel metal hydride, zinc-air, silver oxide,
and/or carbon-zinc. In another embodiment, the potential may be
applied through a potentiostat or some other source of direct
current, such as a photovoltaic cell. In one embodiment, a
potentiostat may be powered by an AC adaptor, which is plugged into
a standard building or home electric utility line. In one
embodiment, the potentiostat may connect with a reference electrode
in the electrolyte solution. Preferably the potentiostat is able to
supply a relatively stable voltage or potential. For example, in
one embodiment, the electrochemical cell is subjected to a voltage
that does not vary by more than 5%, preferably by no more than 3%,
preferably by no more than 1.5% of an average value throughout the
subjecting. In another embodiment, the voltage may be modulated,
such as being increased or decreased linearly, being applied as
pulses, or being applied with an alternating current. Preferably,
the composite thin film electrode may be considered the working
electrode with the counter electrode being considered the auxiliary
electrode. However, in some embodiments, the composite thin film
electrode may be considered the auxiliary electrode with the
counter electrode being considered the working electrode.
[0146] In one embodiment, the composite thin film electrode has a
mass activity in range of 38-50 A/g, preferably 40-48 A/g, more
preferably 42-46 A/g at .eta.=350 mV. The specific potential value
may be 1.45-1.60 V, more preferably 1.48-1.58 V, or 1.58 V vs.
RHE.
[0147] In one embodiment, the method also involves the step of
separately collecting H.sub.2-enriched gas and O.sub.2-enriched
gas. In one embodiment, the space above each electrode may be
confined to a vessel in order to receive or store the evolved gases
from one or both electrodes. The collected gas may be further
processed, filtered, or compressed. Preferably the H.sub.2-enriched
gas is collected above the cathode, and the O.sub.2-enriched gas is
collected above the anode. The electrochemical cell, or an
attachment, may be shaped so that the headspace above the composite
thin film electrode is kept separate from the headspace above the
reference electrode. In one embodiment, the H.sub.2-enriched gas
and the O.sub.2-enriched gas are not 100 vol % H.sub.2 and 100 vol
% O.sub.2, respectively. For example, the enriched gases may also
comprise N.sub.2 from air, and water vapor and other dissolved
gases from the electrolyte solution. The H.sub.2-enriched gas may
also comprise O.sub.2 from air. The H.sub.2-enriched gas may
comprise greater than 20 vol % H.sub.2, preferably greater than 40
vol % H.sub.2, more preferably greater than 60 vol % H.sub.2, even
more preferably greater than 80 vol % H.sub.2, relative to a total
volume of the receptacle collecting the evolved H.sub.2 gas. The
O.sub.2-enriched gas may comprise greater than 20 vol % O.sub.2,
preferably greater than 40 vol % O.sub.2, more preferably greater
than 60 vol % O.sub.2, even more preferably greater than 80 vol %
O.sub.2, relative to a total volume of the receptacle collecting
the evolved O.sub.2 gas. In some embodiments, the evolved gases may
be bubbled into a vessel comprising water or some other liquid, and
higher concentrations of O.sub.2 or H.sub.2 may be collected. In
one embodiment, evolved O.sub.2 and H.sub.2, or H.sub.2-enriched
gas and O.sub.2-enriched gas, may be collected in the same
vessel.
[0148] Several parameters for the method for decomposing water may
be modified to lead to different reaction rates, yields, and other
outcomes. These parameters include, but are not limited to,
electrolyte type and concentration, pH, pressure, solution
temperature, current, voltage, stirring rate, electrode surface
area, texture and nanostructure of the CoVO.sub.x layer, substrate
conductivity, and exposure time. A variable DC current may be
applied at a fixed voltage, or a fixed DC current may be applied at
a variable voltage. In some instances, AC current or pulsed current
may be used. A person having ordinary skill in the art may be able
to adjust these and other parameters, to achieve different desired
nanostructures. In other embodiments, the electrochemical cell may
be used for other electrochemical reactions or analyses.
[0149] In an alternative embodiment, the composite thin film
electrode may be used in the field of batteries, fuel cells,
photochemical cells, water splitting cells, electronics, water
purification, hydrogen sensors, semiconductors (such as field
effect transistors), magnetic semiconductors, capacitors, data
storage devices, biosensors (such as redox protein sensors),
photovoltaics, liquid crystal screens, plasma screens, touch
screens, OLEDs, antistatic deposits, optical coatings, reflective
coverings, anti-reflection coatings, and/or reaction catalysis.
Similarly, in one embodiment, the composite thin film electrode may
be coated with another material. For example, the composite thin
film electrode may be coated with a layer of gold. A gold-coated
composite thin film electrode may then be used for analyte
detection using surface enhanced Raman scattering (SERS).
[0150] The examples below are intended to further illustrate
protocols for preparing, characterizing the CoVO.sub.x films, and
uses thereof, and are not intended to limit the scope of the
claims.
Example 1
Film Electrode Fabrication
[0151] The film electrode fabrication was achieved through AACVD
method. All chemicals were obtained from Sigma Aldrich and were
used as received: Cobalt(II) acetylacetonate (Co(acac)).sub.2),
vanadium(III) acetylacetonate (V(acac).sub.3), methanol (99.9%),
and nitrogen gas (99.99%). The synthesis of Co:V oxide films in a
1:1 stoichiometry was achieved by dissolving 500 mg, (0.19 mmol) of
Co(acac).sub.2 and 500 mg (0.14 mmol) V(acac).sub.3 in methanol (20
mL) in a schlenk tube connected with a vacuum line. The transparent
dark brown solution was stirred for 30 min and solvent was
evaporated under reduced pressure to give a brown solid which was
re-dissolved in methanol (10 mL). The transparent solution was
further stirred for 10 min and was used in AACVD for films
deposition. Prior to the deposition, an FTO glass substrate was cut
to an area of 1.0.times.2.0 cm.sup.2 (W.times.L) and sequentially
washed using soapy water, acetone, and ethanol. The substrate was
then laid horizontally inside the reactor tube and heated up to the
deposition temperature of 475.degree. C. for 10 min to stabilize
the temperature before carrying out the deposition. An aerosol mist
of the precursor solution was generated using a piezoelectric
ultrasonic humidifier. Nitrogen gas was used as a carrier gas to
transport the aerosol to the heated substrate at a rate of 120
cm.sup.3/min. The reactor exhaust was vented into a fume hood. When
the precursor solution and associated aerosol mist had been
completely emptied from the flask, the coated substrate was cooled
under a continuous flow of N.sub.2 gas. The coated substrate was
not removed from the reactor until it reached a temperature of
below 40.degree. C.
[0152] The deposition experiments were carried out for different
time periods such as 20 min, 40 min, and 60 min, and the resultant
film electrodes are named as CoVO.sub.x-20, CoVO.sub.x-40, and
CoVO.sub.x-60, respectively.
Example 2
Film Characterization
[0153] The structural properties of cobalt vanadium oxide thin
films were analyzed by recording X-ray diffraction (XRD) patterns
using a PANanalytical, X'PertHighScore.RTM. diffractometer with
primary monochromatic high intensity CuK.sub..alpha.
(.lamda.=1.5418 .ANG.) radiation. The surface micrographs of the
films were examined using a Lyra 3 .RTM. Tescan, field emission gun
(FEG)-SEM at an accelerating voltage of 5 kV and a working distance
of 10 mm. The Co/V atomic ratios were determined from Energy
dispersive X-ray (EDX, INCA Energy 200.RTM., Oxford Inst.)
spectrometer. X-ray photoelectron spectroscopy (XPS) was done using
an ULVAC-PHI Quantera II.RTM. with a 32-channel Spherical Capacitor
Energy Analyzer under vacuum (1.times.10.sup.-6 Pa) using
monochromatic Al K.alpha. radiation (1486.8 eV) with a natural
energy width of 680 meV. The carbonaceous C is line (284.6 eV) was
used as a reference to calibrate the binding energies.
Example 3
Electrochemical Measurements
[0154] All the electrochemical measurements were performed on a
computer-controlled AUTOLAB.RTM. potentiostat employing CoVO.sub.x
thin film electrodes as the working electrode. A Pt wire shaped
into a spiral (thickness=0.25 mm) was used as the counter electrode
and saturated silver-silver chloride (Ag/AgCl in saturated solution
of KCl) was used as the reference electrode. For long-term
electrocatalysis, a saturated calomel electrode (Hg/HgO) reference
electrode was employed in 0.1 M KOH solutions. However, all the
potentials are referred to reversible hydrogen electrode (RHE)
following the Nernst equation:
E.sub.RHE=E.sub.REF+E.sub.0 REF+0.059(pH).
[0155] Before placing into the electrochemical cell, the platinum
wire was cleaned by immersing in a 20% solution of HNO.sub.3 for a
few minutes following washing with MilliQ.RTM. water. All the
glassware and electrochemical cell were cleaned by boiling in a 1:3
mixture of H.sub.2SO.sub.4 and HNO.sub.3 followed by boiling in
water. The electrochemical cell was then carefully rinsed with
acetone and dried by keeping in oven at 100.degree. C. for 1 hour
as described previously. See Yu, F.; Li, F.; Zhang, B.; Li, H.;
Sun, L., Efficient Electrocatalytic Water Oxidation by a Copper
Oxide Thin Film in Borate Buffer. ACS Catalysis 2015, 5 (2),
627-630, incorporated herein by reference in its entirety.
Electrochemical investigations such as cyclic voltammetry, EIS and
controlled potential bulk electrolysis experiments were performed
in 0.5 M KOH electrolyte solution having a pH.apprxeq.13.6. Water
used to make all the solutions for electrochemical studies was
distilled and deionized using a MilliQ.RTM. system from Millipore.
Linear sweep voltammetry was used in order to find the
overpotentials and current density profiles of the films during
water oxidation reaction whereas the charge transfer resistances
were determined by EIS. The details of measurement parameters such
as the calculation of mass activity and the electrochemically
active surface area (ECSA) are provided herein.
Example 4
[0156] Scheme of Synthesis with Structural and Morphological
Analysis:
[0157] The schematic description for the fabrication of mixed metal
oxide films of cobalt and vanadium is provided in FIG. 10. The
fabrication was performed using an AACVD protocol, the operation of
which is critically related to the solubility of the precursors in
common organic solvents. Moreover, the precursor solutions must be
homogenous, clear, and precipitation free, especially in the case
of mixed metal oxides. Therefore, for the fabrication of Co--V
films, acetylacetonate precursors of both metals were chosen, which
are commercially available and known for their higher solubility in
methanol without using any solubility enhancing reagents such as
trifluoroacetic acid. With this selection of same ligand system for
both metals, the possibility of exchange reactions was ruled out,
which may cause solution inhomogeneity with the passage of time.
This results in a superior particle-particle or particle-conducting
layer connection during direct deposition to force well adhered
films, possibly on a variety of available substrates. The
deposition of these films was done directly on the FTO electrodes
at a relatively low temperature of 475.degree. C. without any need
of further immobilization as in the case of hydrothermal synthesis,
colloidal approaches, or other wet chemistry protocols. See Tan,
C.; Zhang, H., Wet-chemical synthesis and applications of non-layer
structured two-dimensional nanomaterials. Nature Communications
2015, 6, 7873, incorporated herein by reference in its entirety.
Three different films were prepared at deposition times of 20, 40,
and 60 minutes, and this time is multiple orders lesser than wet
chemistry protocols often needing 24-48 hours for the reaction to
complete. These films were correspondingly named CoVO.sub.x-20,
CoVO.sub.x-40, and CoVO.sub.x-60.
[0158] Surface morphology and the corresponding cross sectional
images of the prepared films were investigated by FE-SEM, and the
data is provided in FIGS. 2A-2I. Large area analysis of all the
films (FIGS. 2A, 2D, 2G) shows a uniform and homogenous surface
character, even for the extended deposition time up to 60 min.
However, the morphology of the surface becomes more varied as the
time of deposition increases. It is clearly seen that CoVO.sub.x-20
film attains a spongy character with growth in all dimensions (FIG.
2A), which appeared to be a network of interwoven nanofibers
stacked over each other at a higher resolution scan (FIG. 2B). The
pore size in this case was found to be 3-4 nm with extremely
homogenous distribution. Such a porous and high surface area
structure provides a higher access of the active sites to the
reacting substrates, which is extremely useful for the
electrocatalytic reactions. In CoVO.sub.x-40 film, the networked
surface structure has transformed into nanoflakes (FIG. 2E) which,
with further growth in case of CoVO.sub.x-60, has changed into a
thick continuous film with flakes protruding out of it (FIG. 2H).
However, the imprints of networked CoVO.sub.x-20 structure remained
visible in other two films, showing a coalesced growth. It is
observed that the porous womb-like structure self-propagated into
dense thick layer of CoVO.sub.x. The cross sectional image of
CoVO.sub.x-20 shows that a 1 .mu.m thick film (FIG. 2C) has already
been fabricated in just 20 min, which was then grown into 3 .mu.m
and 4 .mu.m thickness for CoVO.sub.x-40 (FIG. 2F) and CoVO.sub.x-60
(FIG. 2I) respectively. However, this growth pattern has not
disrupted the homogeneity of the formed films as shown by
continuous thickness of all the cross sections.
[0159] FIG. 11 demonstrates the XRD analysis of all CoVO.sub.x
films prepared on FTO substrates at 475.degree. C. The
diffractogram reveals the amorphous nature of all the films even
for the extended deposition time of 60 min. While the FTO substrate
is highly crystalline in nature, its crystalline peaks are
suppressed due to the non-crystalline profile of the prepared
materials in all cases. To confirm the amorphous nature of
CoVO.sub.x, the pristine films of CoO.sub.x and VO.sub.x were also
prepared under similar AACVD conditions using the individual
precursors (i.e., Co(acac).sub.2 and V(acac).sub.3) and their
diffraction patterns are shown in FIGS. 12A and 12B. These patterns
show crystalline peaks of individual CoO.sub.x and VO.sub.x in pure
form. It is an indication that the mixed films thus prepared are
free of crystalline impurities of Co oxide and V oxide and that
only the mixed material is amorphous in nature. At the same time,
this is quite in accordance with the previous reports of amorphous
Co--V mixed oxide materials prepared by hydrothermal methods and
co-precipitation technique, although the materials were synthesized
at more extreme conditions of pressure or temperature. See Liardet
et al. (2018); and Liu et al. (2018). Thus, it may be concluded
that the inherent nature of mixed Co--V oxides is to be amorphous,
although the synthetic schemes in previous reports claim
otherwise.
[0160] The composition and elemental stoichiometry of all the films
were characterized by energy dispersive x-ray (EDX) analysis, and
the resulting spectra are shown in FIGS. 3A, 3D, and 3G for the
samples CoVO.sub.x-20, CoVO.sub.x-40, and CoVO.sub.x-60,
respectively, with respective spectra taken from the regions shown
in FIGS. 3C, 3F, and 3I. FIGS. 3B, 3E, and 3F also show the values
of percent atomicity for both metals in the films CoVO.sub.x-20,
CoVO.sub.x-40, and CoVO.sub.x-60, respectively. The Co:V ratio
found in each case is nearly equal to unity. These data are
provided in tabulated form in Table 1. Further, the presence of
both cobalt and vanadium elements in the film was ascertained by
conducting the EDX mapping. FIGS. 13A-13I indicate the uniform
distribution of Co and V elements in all the films. FIGS. 13A, 13D,
and 13G show FESEM images of CoVO.sub.x-20, CoVO.sub.x-40, and
CoVO.sub.x-60, respectively. FIGS. 13B, 13E, and 13H show the VKal
signal obtained from the regions of FIGS. 13A, 13D, and 13G,
respectively. FIGS. 13C, 13F, and 13I show the CoKal signal
obtained from the regions of FIGS. 13A, 13D, and 13G,
respectively.
TABLE-US-00001 TABLE 1 Atomicities of the two metals and their
atomic ratio in the resulting film. Sample % atomicity Co %
atomicity V Co:V CoVOx-20 9.10 10.58 0.86:1 CoVOx-40 16.62 15.15
1.1:1 CoVOx-60 20.24 19.53 1.05:1
[0161] The amorphous CoVO.sub.x-20 film was further characterized
by X-ray photoelectron spectroscopy (XPS) in order to find out the
oxidation states of the constituent elements. The survey spectrum
so obtained indicated the presence of Co 2p, V 2p and O 1s in the
film. The atomic ratio of cobalt to vanadium (Co:V) determined by
XPS is approximately 16.56:15.95 (1:1) and is consistent with the
metallic ratio of the both elements from the EDX analysis. The high
resolution deconvoluted spectra of these individual elements are
provided in FIGS. 4A-4C. The binding energy 781.1 eV of Co 2p
spectra can be fitted to Co.sup.+2 (FIG. 4A). Moreover, two
satellite peaks are indicated at binding energies of 786 and 788
eV, which are characteristic of high-spin Co.sup.2+. Contrarily, V
2p spectrum has a peak observed at 2p 3/2 (517.5 eV) which
indicates the V.sup.+4 oxidation state (FIG. 4B). Furthermore, the
unsymmetrical peak O 1s can be further split into three peaks. The
high resolution signals at binding energies of 531.5 eV and 530.0
eV in the O 1s spectrum represent oxygen atoms in hydroxyl group
and oxide group, respectively. This demonstrates a characteristic
feature for O that is bonded to a metal in metal oxides. The
binding energy peak at 532.3 eV is again a feature of oxygen atoms
in carbonate group (FIG. 4C). Although the XRD studies could not
exactly reveal the chemical formula of the deposited material due
to the non-crystalline nature of the fabricated material, on the
basis of XPS studies the oxidation state of Co and V atoms existing
in the binary oxide system can be demonstrated. These XPS studies
also show a good agreement with the CoVO.sub.x materials fabricated
by other synthetic strategies. See Liardet et al. (2018); and
Thorat et al. (2018).
Example 5
Electrochemical Water Oxidation Studies
[0162] Directly deposited amorphous catalytic films of CoVO.sub.x
on the FTO substrates were used for water oxidation studies without
any further modification. FTO substrates are much less conductive
than the reported glassy carbon and nickel foam, carbon foam, and
gold foam materials used for CoVO.sub.x immobilization, but are
less costly, easily available, and scalable for large area
applications. See Liardet et al. (2018); Thorat et al. (2018); and
Liu et al. (2018). The water oxidation experiments were performed
in 0.5 M KOH using three electrode configuration under forward
potentials sweeps, and the linear sweeps voltammetry (LSV) profiles
were obtained. FIG. 5A indicates these profiles for all three Co--V
films at a scan rate of 10 mV/sec. All the data was compared at a
current density of 10 mA/cm.sup.2, which is often considered as a
reference for providing 10% efficiency in water splitting
reactions. Here, CoVO.sub.x-20 film showed remarkable performance
in terms of onset overpotential (i.e., 270 mV), overpotential at 10
mA/cm.sup.2 (i.e., 310 mV), and current density reaching to a value
of 160 mA/cm.sup.2 only at an overpotential of 410 mV. This
catalytic performance of CoVO.sub.x-20 is even higher compared to
the other two films which both show a similar onset potential but
have different overpotential at a current density of 10
mA/cm.sup.2. Better performance of CoVO.sub.x-20 can be justified
on the basis of porous nanofiber film structure indicated by the
SEM topography (FIGS. 2A and 2B). This porous structure with spongy
appearance facilitated the higher number of catalytic sites to be
accessible for the reaction to proceed. As a consequence, the
overpotential is reduced, and the current density jumps to higher
values at lower potentials. With an increase in deposition time,
the porosity of the film starts deteriorating, and the surface
structure becomes more compact, as evident from FIG. 2E. Thus, less
porous flakes of the material are formed, with an increase in
overpotential (i.e., 350 mV and 369 mV for CoVO.sub.x-40 and
CoVO.sub.x-60, respectively as shown in FIG. 5B. The current
densities of these films were also shifted towards lower values.
The performance of the CoVO.sub.x-20 film was also compared to its
forming materials and to the film structures reported using other
fabrication strategies. The overpotential for CoVO.sub.x-20 film is
much smaller than the reported values of individual metal oxides of
the combination and is lower than many cobalt vanadium oxide
catalysts, such as Co.sub.2V.sub.2O.sub.7 (340 mV),
Co.sub.3V.sub.2O.sub.8 (359 mV), and Co.sub.3V.sub.2O.sub.8
nanoroses (391 mV). See Peng et al. (2017); and Xing, M.; Kong,
L.-B.; Liu, M.-C.; Liu, L.-Y.; Kang, L.; Luo, Y.-C., Cobalt
vanadate as highly active, stable, noble metal-free oxygen
evolution electrocatalyst. Journal of Materials Chemistry A 2014, 2
(43), 18435-18443, each incorporated herein by reference in their
entirety. This value is also comparable to the values reported for
amorphous cobalt vanadium oxide on glassy carbon (330 mV), nickel
foam (260 mV), and gold foam (215 mV), although the substrates used
herein are FTO and are much less conductive than metallic foams.
See Liardet et al. (2018); and Liu et al. (2018), as cited
previously. A notably higher current density is also achieved
without a substantial increase of overpotential, which makes this
AACVD strategy more direct and viable for catalyst production.
[0163] An effect of change in the scan rate on the CoVO.sub.x-20
film was also studied in FIGS. 6A and 6B. It was found that
increasing the scan rate from 1 mV/sec to 100 mV/sec shifted the
onset potential to more negative potential values. However, there
is no substantial change in the overpotential for 10 mA/cm.sup.2
current density as this value shifted from 310 mV at a scan rate of
10 mV/sec to 308 mV at a scan rate of 1.0 mV/sec, as shown in FIG.
6B. Notable here is that the current densities are higher at low
scan rates, reaching a value of 175 mA/cm.sup.2 for 1 mV/sec at an
overpotential of just 380 mV. With a direct and rapid synthetic
strategy taking only minutes for the whole preparation process, and
in view of the less conductive behavior of the FTO substrates, this
water splitting performance is quite remarkable.
[0164] In order to study the sustainability and the consistent rate
of water oxidation reaction, Tafel plots were drawn for all the
prepared catalyst films within the linear regions of the current
voltage curves at a scan rate of 10 mV/sec and were fitted into the
Tafel equation. This analysis provides an indication of whether the
catalyst can operate over a narrow potential range while producing
high current density. A small Tafel slope is an expression of
well-balanced kinetics during catalysis. See Shinagawa, T.;
Garcia-Esparza, A. T.; Takanabe, K., Insight on Tafel slopes from a
microkinetic analysis of aqueous electrocatalysis for energy
conversion. Scientific Reports 2015, 5, 13801, incorporated herein
by reference in its entirety. These plots with their slope values
are shown in FIG. 7. All three catalytic films presented here
demonstrated the enhanced kinetics while displaying appreciably low
value of Tafel slopes, however, the film formed with 20 min
deposition has a very large linear range with a low slope of 75
mV/dec. The CoVO.sub.x-40 film also showed a similar linear range
but with a higher value of the slope (i.e., 84 mV/dec).
CoVO.sub.x-60 film, on the other hand, staredt deviating from the
linear range at log value of 1.4 with a slope of 149 mV/dec. The
Tafel slope value for the CoVO.sub.x-20 at a scan rate of 1.0
mV/sec was even shifted to 62 mV/dec with a linearity of response
even beyond a logarithmic value of 2.2. This indicates that
catalytic performance is dependent upon the porosity and surface
structure of the material, and so is the kinetics of the reaction
which is more visible at lower scan rates. An open structure and
porous morphological features of the CoVO.sub.x-20 film catalyst
supports the fast mass transfer and boosts the electron transfer
without undergoing any scattering losses, as a higher number of
accessible catalytic sites are readily available.
[0165] For a quantitative analysis of the film properties, mass
activities of the catalytic films were determined at 1.55 V vs RHE
as provided in Table 2. This data also corroborates the Tafel plots
where the smaller Tafel slopes are linked to the higher mass
activity in the same order. The higher mass activity of the
CoVO.sub.x-20 (i.e., 43.4 A/g) as compared to the other two films
further demonstrates the high mass performance of 20 min deposition
of the material. Here, a longer deposition and higher mass loading
does not contribute to the catalytic activity, rather the
microstructure of the film and the available catalytic sites are
responsible for the OER. For an estimation of the available
catalytic sites, measurement of the active surface area can be
regarded as an important factor which was numerically assessed from
the double-layer capacitances measurements. For that purpose,
charging current density differences in a potential window of
non-faradaic region were plotted against scan rates in FIG. 14, and
the slopes were divided by the electrode area to get estimated ECSA
values in units of mF/cm.sup.2. A significantly higher linear slope
value of CoVO.sub.x-20 film (i.e., 16.87 mF/cm.sup.2) as compared
to CoVO.sub.x-40, which has a value of 10.12 mF/cm.sup.2, and
CoVO.sub.x-60 which has a value of 7.06 mF/cm.sup.2, indicates that
the CoVO.sub.x-20 film has a higher number of active sites
available, thereby making the catalytic reaction kinetically
favorable.
TABLE-US-00002 TABLE 2 Summary of electrocatalytic activity for
Co--V mixed oxide thin film electrocatalysts. Current Density at
Tafel Mass .eta.@10 mA/cm.sup.2 .eta.@100 mA/cm.sup.2 1.60 mV vs
RHE Slope Activity ECSA Sample [mV] [mV] [mA/cm.sup.2] [mV/dec]
[A/g] [mF/cm.sup.2] CoVOx-20 310 370 48 75 43.4 17.63 CoVOx-40 350
480 25 84 35.9 10.12 CoVOx-60 369 590 13 134 29.2 7.06
[0166] The conductivities of the sample films were estimated using
impedance spectroscopy, showing a characteristic depressed
semicircle of an OER charge transfer reaction for all the films.
However, it was found that CoVO.sub.x-20 film exhibits highest
conductivity which is an indication of its lowest charge-transfer
resistance compared to other two films as detailed in FIG. 15. FIG.
15 shows a Nyquist plot for CoVO.sub.x-20, CoVO.sub.x-40, and
CoVO.sub.x-60 films at an applied potential of 1.48 V vs. RHE in
the frequency range of 0.1 Hz to 100 KHz. For each EIS analysis,
data were fitted employing Randles circuit with Nova software. If
this resistance is compared to the materials immobilized on gold
foam (i.e., 10-25 ohm), nickel foam (i.e., 1.0-1.4 ohm), glassy
carbon (i.e, 0.6-0.8 ohm), and carbon foam (4.0-8.0 ohm), it is at
least ten times higher because of direct deposition on FTO
substrate. See Liu et al. (2018); Liardet et al. (2018); and Thorat
et al. (2018). Thus, the method of AACVD deposition can be regarded
as highly efficient for water oxidation chemistry. Among the three
AACVD films, the higher conductivity of the CoVO.sub.x-20 may have
originated from its thinner structure with the network of fibers
having an enhanced number of active sites available. The larger
number of catalytic sites transforms more and more of the cobalt
species into their active form, in real time during the catalysis,
thus promoting the charge transfer process. Consequently, the
conductivity as well as the catalytic performance of this film is
significantly improved.
[0167] In addition to the enhanced catalytic activity with low
conducting substrates, the CoVO.sub.x films also exhibited highly
desirable catalytic durability and stability. For more than 5 h of
constant anodic polarization in each case of 20 mA/cm.sup.2 and 100
mA/cm.sup.2 current densities, only a moderate increase of 10-30 mV
in overpotential was required for all three fabricated films, as
shown in FIG. 8A. This nominal shift in the overpotentials is
caused by the accumulation of very high density of oxygen covering
the active sites at the electrode surface. Bubbles in the form of a
rich continuous stream of oxygen bubbles was also visible during
the electrocatalytic experiments owing to the high rate of oxygen
production at these electrodes. Moreover, the CoVO.sub.x-20
electrode exhibited only a small loss of activity after 500 CV
cycles as shown in FIG. 8B. All these performance parameters of the
fabricated films can only be attributed to their excellent
catalytic activity under the employed conditions.
Example 6
Calculation of Different Electrocatalytic Parameters
Electrochemical Impedance Spectroscopy
[0168] EIS analysis was carried out to get more insight into
electrochemical kinetics for all the thin film electrocatalysts.
The data was recorded at an applied potential of 1.49 V vs. RHE
considering the faradaic region of cyclic voltammogram to
investigate charge transfer resistance at the so-called
electrode-electrolyte double layer.
Mass Activity (MA) [mAmg.sup.-1]
[0169] The loading normalized current density or mass activity is
calculated according to the following formula:
MA = J @ .eta. = 0.35 V Active mass of catalyst . ##EQU00001##
Here, J is current density in mAcm.sup.-2 at specific potential
value. 1.58 V vs. RHE was chosen as the specific potential value.
Electrochemically Active Surface Area (ECSA) [mF/Cm.sup.2]
[0170] Electrochemically active surface area (ECSA) was calculated
using CV mode by calculating double layer capacitance employing the
following formula:
ECSA = CDL Cs ##EQU00002##
First of all, the non-faradaic region (somewhere in between the
oxygen and hydrogen region) in the CV was identified by visual
analysis of cyclic voltammetry data assuming that all the current
in this potential range was due to the double layer charging. Under
this potential range the CV was run at different scan rates (5
mVs.sup.-1, 10 mVs.sup.-1, 20 mVs.sup.-1, 50 mVs.sup.-1). The
charging current (Ic) is calculated by identifying a middle
potential range, which was 0.955 V vs. RHE, and the current
associated with this potential range was considered as capacitive
current or charging current. A plot of scan rate versus capacitive
current was constructed, and the slope of this calibration curve
gave a value of double layer capacitance per unit area, which
serves as the estimation of ECSA.
Example 7
Proposed Mechanism
[0171] All of the above measurements as well as related works have
led to a proposed mechanism of the catalytic process and the role
of vanadium moieties in the system. See Ehsan et al. (2018); Liu et
al. (2018); and Xing et al. (2018), as cited previously and
incorporated herein by reference in their entirety. As shown in
FIG. 9, whether V is present or absent, the first few steps of the
process are same which involves the activation of Co sites in the
alkaline medium. The reaction may begin with the adsorption of
water and discharge of hydrogen and electrons to form adsorbed
hydroxyl groups on the surface. These hydroxyl groups may react
with more hydroxyl ions under the forward potential sweep, thereby
leaving the oxygen atoms adsorbed onto the film substrate. In the
absence of V, the reaction continues with the generation of OOH
groups at the surface, which is considered as the rate limiting
step. Then in the next step, a water molecule leaves the surface
leaving an oxygen molecule adsorbed on the surface. This oxygen is
then removed, completing the oxygen evolution reaction in the last
step. However, in the presence of V, which is capable of switching
its ground state, thereby modifying metal-metal and antibonding
interactions in the catalytic cluster, the adsorbed oxygen species
on the surface can form oxobridged entities among the neighboring
oxygen atoms. This scheme of operation benefits the catalytic
reaction in two ways. First, this oxobridged state is relatively
more active, which leads to a thermodynamically favorable
generation of a hydroperoxo intermediate and kinetically faster
O--O bond formation, although it still remains a rate limiting
step. Second, the oxobridged entities can also act as a catalyst,
facilitating the reactions forming adsorbed hydroxyl and isolated
oxygen species on the catalytic site. Thus, a favorable catalytic
cycle is achieved leading to better performance of mixed oxide
films. The presence of V in Co-oxide materials has also been shown
to have a decreased overpotential at Co-active sites adjacent to V.
However, the V-active sites show increased overpotential. This
corresponds exactly to the volcano plots described earlier which
show that a mixed oxide configuration can stabilize the bond
strength and decrease the overpotential. This enhanced catalytic
impact of V on the neighboring Co atoms can be ascribed to the
coordination environment of Co atom and its modification caused by
the lattice mismatch when V atoms are also present close by. In
this manner, the Co-active sites can attain favorable water
oxidation energies and the enhanced activity of OER catalysis is
achieved.
Example 8
Observations
[0172] Amorphous CoVO.sub.x films fabricated herein have shown a
highly efficient catalytic character. Further, the target of
obtaining well adhered and uniform films of all the materials was
achieved directly on the substrate surfaces with changing
morphological and catalytic character with variations in deposition
times. The CoVO.sub.x-20 film, due to its peculiar networked
structure exposing larger number of catalytic sites, its high mass
activity, larger ECSA, and low charge transfer resistance
demonstrated lower overpotential, higher current density, and lower
Tafel slope as compared to CoVO.sub.x-40 and CoVO.sub.x-60. These
numbers were comparable to only a few CoVO.sub.x materials
previously reported, however, a clear advantage was the formation
of easily scalable films in just 20 min with a one-step procedure
without any immobilization required. Moreover, it was unprecedented
that the CoVO.sub.x films deposited on less conductive FTO
substrates showed such a high catalytic activity. This
characteristic paves the way for building and understanding new
catalytic materials, using them in various applications besides
water splitting reactions, and then moving towards commercial
products.
[0173] A rapid one-step aerosol assisted chemical vapor deposition
(AACVD) method was employed to synthesize amorphous and highly
active Cobalt Vanadium mixed oxide catalysts (CoVO.sub.x) directly
over FTO substrates. Morphological and structural characterizations
made by FE-SEM, XRD, EDX, and XPS revealed the formation of pure
phase amorphous films with a gradual variation of topography as a
function of deposition time. The most active of those films,
CoVO.sub.x-20, was obtained in 20 min deposition, showing a spongy
network of interwoven nanofibers with a homogeneous distribution of
3-4 nm pores, achieving an overpotential of 308 mV at a 10
mA/cm.sup.2 current density. A much higher current density of 175
mA/cm.sup.2 could be achieved just 380 mV of overpotential with a
Tafel slope as low as 62 mV/dec for this whole range while
exhibiting long term stability. Mass activity, EIS data, and the
estimation of ECSA all proved this high catalytic performance of
CoVO.sub.x-20 which is unprecedented for a low cost, up-scalable,
and relatively low conductive substrate such as the FTO used
herein. The findings not only highlight the benefit of using AACVD
in preparing two-dimensional amorphous catalysts, but also prove
the high efficiency of CoVO.sub.x materials thus obtained as
outlined in a plausible reaction mechanism.
* * * * *